I was asked about doing something like this a while ago, said "Yeah, that sounds like a good idea," and then, um, didn't do it, sorry about that. Still, better late than never, right? This is some basic background information about small-arms ammunition as it it most commonly encountered nowadays. Some of it will have appeared elsewhere in these pages, other bits haven't come up so far that I can recall.

First, some very brief historical background. The self-contained cartridge as we know it today originated in the mid-1800s. "Cartridges" by that name had existed for a while by then, but the word denoted something different before about 1860. Prior to the invention of the self-contained metallic case and integral primer (the latter of which I have always assumed was a logical extension of the percussion cap), a cartridge in the firearms sense was basically a prepared packet of all the stuff you needed to load a muzzle-loading musket or rifle.¹ They were usually made of paper, and the idea was that they would be taken apart and the parts put into the gun separately. Often the paper was used as wadding between the powder charge and the bullet, so all of it got used, but the early ones were not meant to be put into the firearm as a unit.

(Later on that would sometimes change. Needle guns, for instance, were breechloaders that used paper cartridges—hadto, since the needle needed to be able to pierce the cartridge case to reach the primer inside. Those didn't stay in vogue for very long, though. Nowadays it's possible to make cap-and-ball revolver cartridges out of flash paper, so that it burns up along with the powder charge, to make reloading more convenient, but they're not necessary to make the guns function like needle guns' paper cartridges were.)

The early metallic cartridges were made from rolled brass or copper foil, which meant that they were flimsy, easily damaged, and not reusable. Not until someone worked out how to draw solid brass into a cylinder open at one end would it be really practical to make a reusable metallic cartridge case. Once someone did, though, the advantages of the system were so obvious that it quickly became The Way It's Done, and remains so to this day. Many designers have fooled around with "caseless" ammunition—back in the 1980s you would often find firearms futurists and cyberpunk roleplaying game sourcebook authors confidently predicting that the metallic cartridge would be Over by the turn of the century—but it's never really worked or offered any worthwhile advantage.

Anatomy

The photo above (showing unfired ammunition that's just been dumped out of an SKS carbine) gives a decent idea of the anatomy of a typical—not to say practically ubiquitous, these days—centerfire cartridge (7.62x39mm Soviet). There are four basic parts.

A bit paradoxically, the part of the case that's at the back when the cartridge is in its usual position in the gun is the head. Most cases have some information stamped on their heads (unsurprisingly called the headstamp); in commercial ammunition this is usually the caliber and often the brand, while military-made ammunition more commonly has random-looking code numbers not comprehensible to anyone outside the logistics and supply systems where it originated.

The cartridge we see here is called a rimless cartridge, even though it has an obvious rim. It's called that because the rim is the same diameter as or smaller than the case body ahead of it, and has an extractor groove for the firearm's extractor to grab hold of instead. We can compare this to an earlier rimmed cartridge:

Notice the obvious protruding rim on the top cartridge. This has its uses (which we'll get to in a minute), but rimmed cartridges aren't very good for use in repeating, and particularly in automatic, firearms. Anytime you have a bunch of rimmed cartridges in a vertical magazine together, they tend to get in each other's way a lot. Magazine Lee-Enfield rifles, for instance, commonly suffer from a phenomenon called rim lock, which is just what it sounds like, if their loading clips aren't arranged precisely right. In a belt-feed system, rimmed cartridges have to be pulled backward out of the belt and then sent forward into the chamber, making the loading mechanism more complicated. Repeater-wise, they only really work well in revolvers and horizontal tube magazines.

Anyway, moving on. In the center of the case head, that flat bit—best seen on the bottom-most cartridge in the SKS unloading photo—is the primer. This is the part that actually sets off the cartridge. It's made of an impact-sensitive explosive that detonates when it's struck sharply by the gun's firing pin. There's a hole in the case head underneath it, so that the spark of its detonation can go into the main body and ignite the powder charge within, setting off the cartridge.

Primers were probably the trickiest part of developing the self-contained cartridge. They have to be stable enough that they won't go off in the course of ordinary handling, and to have extended shelf lives (that they don't get unstable and tetchy toward the end of, like e.g. dynamite), and yet sensitive enough that they'll explode reliably when struck with the kind of forces available to the firing mechanisms of small arms. And those mechanisms can't be set up to hit them too hard, or they run the risk of perforating the primer entirely, which vents the combustion of the main charge back toward the firing pin. That's bad. You don't want to compromise the head of the cartridge.

Add to that, for decades after they were first developed, the chemicals used in ammunition primers were both toxic (they had fulminate of mercury in them) and corrosive (their combustion byproducts were hygroscopic, that is water-attracting, salts). This meant firearms that used them were prone to rusting in hard-to-reach places (like the bores and the inner workings of the firing mechanisms) if not scrupulously cleaned afterward. This wasn't a maintenance nightmare on par with the ungodly mess left behind by black powder, but it's still a hassle, and something to watch out for when buying elderly ex-military arms. If you buy a Mosin-Nagant or SMLE today, it has almost definitely had a lot of corrosive ammunition put through it, and you had best hope that its prior owners were all diligent about cleaning.

(Even today, if you use surplus military ammunition dating to before about 1950 for Western ammo or the early 1970s for the Soviet bloc, it's almost certainly got corrosive primers and you're going to have some cleanup to do. For that matter, a lot of brand new Russian commercial ammunition that claims it's non-corrosive on the box isn't really, at least not by Western standards. Yeah, I'm looking at you, TulAmmo, don't play innocent.)

While we're talking about rims and primers, this is a good place to mention the other common ignition system of today, rimfire ammunition, which is just what it says on the tin. Rather than having a centrally located primer, rimfire ammunition has the priming compound rolled into the cartridge rim. The firing pin mashes part of this rim against the mouth of the chamber, setting it off and causing the spark to fire sideways into the main charge.² The bottom-most cartridge in the photo below is rimfire:

(Obviously, rimfire ammunition is always rimmed.)

The advantage to rimfire ammunition is that it's cheaper and easier to mass-produce, since there's no separate primer to worry about. On the other hand, it doesn't ignite as reliably as centerfire ammunition, its empties can't be reloaded, and because it has to be rimmed, it inescapably presents all the usual problems of rimmed ammunition to feed systems.

Anyway, moving on. The next part of the cartridge is the body (sometimes also called the hull, though that's more commonly applied only to shotgun shells), which is pretty self-evident. This can be straight or tapered; many rifle and a few handgun cartridges are also bottlenecked, the derivation of which should also be pretty obvious. In a bottlenecked cartridge, the part with the steep inward slant is called the shoulder and the smaller-diameter bit after it is the neck.

Finally, out at the business end, is the bullet. In technical parlance, this term only ever refers to the actual projectile, which is why firearms pedants get so annoyed when, e.g., television reporters refer to whole cartridges as "bullets" (or, even more amusingly, empty cartridge cases as "used bullets", which I've seen more than once). Strictly speaking, a bullet is a tapered projectile of some kind; a spherical projectile is a ball if it's alone or shot if there's more than one in a single load.

Beyond that distinction, bullets come in a lot of different shapes, but only a couple are very common in civilian ammunition these days. There's the classic flat-based, round-nosed bullet, commonly found in handgun ammunition. Modern rifle ammunition usually has what's called a spitzer bullet (from the German Spitzgeschoss, "pointed projectile"), which is longer and pointier, for reasons of superior ballistics (handgun ammo is usually going slow enough that it doesn't need to worry about these things). There are also hollowpoint bullets of both basic shapes, which are designed to increase in diameter when they hit something and thus transfer their kinetic energy more effectively into the target. If you read up on this you will find all sorts of ghastly musings about "hydrostatic shock effects" and "wound channel ballistics" and we're not going to get into any of that here, except to note that ammunition advertisements have always had a certain air of breathless self-congratulation about the lethality of such things. There used to be an ad for hunting hollowpoints with the cheerful strapline, "THE DEADLIEST MUSHROOM IN THE WOODS."

Most bullets are made primarily of lead (hence any number of common figures of speech about getting shot, to be filled full of lead, to have lead poisoning, and so on), because it's easy to work with and very dense. Because lead is quite soft and melts at a fairly low temperature, modern high-velocity cartridges often apply forces and temperatures too great for plain lead bullets to withstand without leaving bits of themselves behind in the rifling (referred to as lead fouling or simply leading), which is undesirable, so it's very, very common nowdays for bullets to be partially or completely jacketed in some harder metal, usually copper. Depending on the manufacturer, jacketed bullets may be labeled "full metal jacket" (hence the title of the film), "total metal jacket", or some variation thereof. The military shorthand for FMJ ammunition, hardball, is commonly used in converstaion but usually not found on the box.

Hollowpoint rounds for handguns used to be notorious for not working very well in semiautomatics, because a lot of early semiautos were designed as military pistols, and armies almost universally use hardball (hollowpoints having been outlawed in military use by the Hague Convention of 1899).³ Hollowpoint bullets are usually not quite as long, and they're obviously not rounded at the end, so they don't play well with many semiautos' feed systems. Nowadays, more cleverly designed hollowpoints and semiauto handguns mean that there are lot of the latter that will work with at least some of the former, but it often takes a bit of hunting around to find the right match.

There are also oddball bullet types you may see the names of from time to time. Wadcutter bullets, for instance, are plain lead, cylindrical and flat on both ends; they take their name from the fact that they were assumed to make more precise holes in paper targets for competition shooting. There is also the semi-wadcutter, which is a wadcutter that tapers a bit up front, for better aerodynamics, but is still flat at the tip. Semi-wadcutters are sometimes partially jacketed on the cylindrical part, to make interacting with rifling cleaner. Some revolver shooters used to think (and a few probably still believe) that wadcutters are Especially Lethal because of their shape. This is probably not true, but you will never win an argument with anyone, ever, over whether any given cartridge or bullet type is more or less lethal than another comparable one, so never mind.

This is probably a good place to define another term you might've seen in a book or movie at some point, the so-called dum-dum bullet. This was an early form of expanding bullet, developed in 1896 for British forces in India. The idea was basically to take a half-jacketed .303 British bullet and cut a crosshatch pattern in the exposed soft-lead nose, so that it expanded more readily when it hit something (or, well, let's be realistic, someone). In the charming way in which these things were often done by the British in those days, this was customarily reserved for use against non-Christian opponents. The dum-dum gets its name from the fact that it was developed at, I kid you not, Dum Dum Arsenal (Dumdum is a town near Calcutta).

Though they were outlawed in war within three years of being invented, dum-dum bullets got into the common parlance anyhow as a shorthand for any soft-pointed or expanding round, particularly one that had been doctored specifically (and perhaps illicitly) to increase its tendency toward expansion. American organized criminals, in particular, were often accused of using them in gangland assassinations. These days it's not usually applied to ammunition that was specifically designed to be expanding, a lot of which is readily available on the civilian market for hunting and/or "defensive" purposes.

That's probably enough for one post. Next we'll talk about nomenclature.

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
Ceterum censeo Carthaginem esse delendam.

¹ A rifle has grooves cut into the barrel to impart a stabilizing spin to the ball or bullet; a musket is smoothbored. Although in the early days of rifling it wasn't uncommon for a muzzleloading rifle to be called a "rifled musket". But that's another show.

² This, by the way, is why dry-firing a rimfire gun (i.e., actuating the firing mechanism on an empty chamber) is generally not a good idea. Because the firing pins of rimfire guns are designed to smack part of the cartridge rim into the chamber mouth, firing them without a cartridge in there can drive the firing pin directly into the chamber mouth. Over time, this will either wear down (or break outright) the firing pin, or peen a depression into the chamber mouth. The latter can lead to ignition failures, as the cartridge rim can deform into the depression rather than being crushed. In either case the gun no longer works, and if you've damaged the chamber, you've pretty much permanently ruined the whole barrel (they're usually one part).

With a centerfire system, you still may damage the firing pin (they're often designed with the assumption that they'll be decelerated by hitting a primer rather than just going straight to the end of their length of travel), but you don't stand to ruin the gun.

³ This is why the stainless steel M1911A1-type pistol prominently featured in the Hitman: Agent 47 video games is called the "Silverballer". It's based on a real-life 1911 clone called the Hardballer, formerly made by Arcadia Machine & Tool. Like virtually all 1911s, the real-life AMT Hardballer was designed to use FMJ ammunition. A tricked-out long-slide Hardballer is also featured in The Terminator, as one of the guns the T-800 steals from the pawn shop.

The exception is that metric cartridge names usually include the length of the case as well as its intended bore diameter in the full designation. This sometimes gets left off in common parlance—for instance, the "by 19" part is often left out of "9×19mm Parabellum"—but it's part of the official names, as set down by standards organizations like the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) in the US and CIP (the Commission internationale permanente pour l'épreuve des armes à feu portatives, "Permanent International Commission for the Proofing of Small Arms") in Europe.

No English-measure cartridge name I can think of includes the length directly, though some old black-powder cartridge names imply it by specifying the volume of powder the case is meant to contain. A second number in the name of a cartridge specified in an English diameter could mean any one of several things. It might mean the aforementioned black powder volume measure (.45-70 Government); it might mean the weight of the specified bullet in grains (.38-200, the British revolver cartridge, a variant of .38 S&W); it could mean the expected weight in grains of smokeless powder (.30-30 Winchester); it might even be the year of standardization (.30-'06 Springfield, the common name for the US Government "Cartridge, Ball,¹ Caliber .30, Model of 1906").

In either case, metric or English, there are usually two or three pieces of information being conveyed by the name of a cartridge. The first, which virtually all cartridge names include, is the caliber—that is, the nominal diameter of the bore.² This is the most critical piece of information that is needed about any given cartridge. The rest of the name is generally to distinguish one cartridge of a particular bore diameter from others that use the same-nominal-size projectile, but are different in other ways.

In metric cartridges, the first extra piece of information is usually the length of the cartridge, as previously noted. Sometimes the rest of the extra information includes the name of the manufacturer that originally developed the cartridge (.30-30 Winchester) or its country of origin (.303 British). Often it will contain some adjective meant to convey something about the size and/or power of the cartridge, as with .38 Special (the "special", as in longer and more powerful, successor to .38 Smith & Wesson, and indeed originally .38 S&W Special), .357 Magnum (named after the "magnum" size of wine bottle, to convey its bigness relative to .38 Special), or .450 Nitro Express (combining "nitro", an indication that it's meant for smokeless powder, with "Express", an old big-game name for a high-velocity loading).

Often, cartridges popularly associated with a specific firearm will be referred to with that firearm's name. This is sometimes official marketing (e.g., the .25, .32, .38, .380, and .45 Automatic Colt Pistol cartridges) and sometimes shooter convention that ends up being put on the box for convenience's sake (7.62mm Tokarev). 9x19mm Parabellum is interesting in that it has two such names, both of which are based on what people called the pistol it was developed for. "Parabellum" was the pistol's official brand name, but virtually everyone calls it the Luger, which is why you will also see the very same ammunition sold as 9mm Luger.

Some cartridges also have the "NATO" tag, most notably 7.62x51mm NATO and 5.56x45mm NATO. These are so called because they were standardized for use by members of the North Atlantic Treaty Organization; 7.62 NATO (commonly conflated with .308 Winchester, though they are in fact slightly different and not universally interchangeable) is the agency's "full-power" rifle cartridge, while 5.56 NATO (often conflated with .223 Remington, ditto) is the "intermediate" rifle/LMG cartridge of choice for participating armies. There is a shit-ton of politics bound up with both of these rounds and we're not going to get into that here.

You will also find this naming convention used to differentiate cartridges which have very similar or identical case geometries, but differ in their powder loading and/or standard bullet weight (and thus their pressure profiles). Thus, 7.65x25mm Borchardt begat both 7.65x21mm Parabellum (same bullet, slightly shorter case) and 7.63x25mm Mauser (same case, slightly smaller bullet), and ultimately 7.62x25mm Tokarev (same case, virtually the same bullet as the Mauser round but measured with the Russian rather than Western European system, hotter powder charge). It can be dangerous to mix any of those up and it's very easy to do if all you have is the digits. The names help to keep everything straight.

(You'll note, by the way, that there are a hell of a lot of both pistol and rifle cartridges with bore diameters in the 7.62–7.65mm range. This is not coincidental; it so happens that that general size, roughly .30-caliber by American standards, was seen as a bit of a sweet spot by ammunition designers for much of the 20th century. 9mm/.38-caliber is a similarly common point of reference, albeit mostly for handguns.)

There is an incredible amount of variation within these parameters, and I'm sure there are a few that step entirely outside them. For all I know, there's at least one cartridge out there in the profusion of different ones developed over the last ~150 years that doesn't even specify its bore diameter, thus providing an annoying way for pedants to say "well actually..." when someone like me says that they always start with that. I mean, there are exceptions to everything in this life. There's even been a kind of handgun ammunition that isn't round. :)

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
Ceterum censeo Carthaginem esse delendam.

¹ Short for "hardball", that is, full metal jacket. See post #0.

² I say "nominal" because there is often some variation, particularly with really venerable cartridges that date to a time when these things were not as precisely measured as they tend to be today. For example, the modern .38-caliber bullet's actual diameter is .357", which dates back to the .36-caliber cap-and-ball Colt revolvers of the 1850s. In this case the ".38" comes from the cartridge case diameter, which has to do with the way .36-caliber Colts were converted to fire cartridges in the 1870s.

Also, sometimes the bore diameters in metric cartridge names get rounded off for convenience's sake, as with 7.92mm Mauser getting shortened to "8mm Mauser", which is easier to say.

Ironically, the failure of caseless ammunition to be widely adopted has to do with metallic cartridges being The Way It's Done. More specifically there are three key issues that have impeded efforts to adopt caseless ammunition: heat, handling, and cost.

The first is the hurdle that has dogged most efforts over the past 40+ years to adopt caseless ammunition because it's something that we take for granted in a metallic cartridge. As noted in the page about open and closed bolt weapons, the firing of a cartridge produces heat as well as propulsive force. A large portion of that heat is absorbed by the cartridge itself and then discarded when the cartridge is ejected from the chamber, while the remaining fraction is absorbed by the chamber itself. This build-up of heat has to be managed carefully, as otherwise it can lead to "cook off" or the auto-ignition of the propellant in a chambered cartridge. In order to avoid this problem in caseless ammunition, designers usually have to adopt "high-temp" propellants that can be pressed into shape but only ignite when the primer is struck.

This leads to the second problem of handling, specifically the loading of said ammunition. The adoption of first paper and then metallic cartridges was driven largely by the ease by which they could be loaded into a firearm without too many complicated steps. The easier it was to handle them, the quicker the firearm could be made ready to fire again. Likewise paper gave way to metal as the realities of warfare drove the need for a cartridge that would readily fire whether wet or dirty. This is where the same "high-temp" propellants become an issue, as the lack of casing means they are exposed to the elements as well as the rough treatment of a battlefield. Likewise the metallic cartridge provides a gas seal when fired, ensuring that the propellant gases are channeled into driving the bullet forward and out of the firearm. In a firearm using caseless ammunition, the chamber and bolt must be specially designed to provide this gas seal. And rounds must be kept sealed, usually in box magazines that are loaded whole into the rifle, preventing soldiers from "topping off" partial magazines or carrying ammunition "loose."

Both of these problems lead to the final one of cost, namely that all the special considerations that go into creating a caseless round that can be fired reliably without danger of "cook-off" and the firearm to fire it properly leads to a very expensive project. As the technology to create and handle metallic cartridges has been around for decades, they can (depending on the round) be produced by the thousands or millions for relatively little cost. By contrast, caseless ammunition is a matter of "reinventing the wheel" and so carries all the inherent costs of research and development before the mass production stage is even reached. To give an idea, we can look at the Heckler & Koch G11, the best known example of a project to develop a mass produced assault rifle for a modern military that utilized caseless ammunition.

The project began in 1969 as a NATO study into a second small round ammunition standard (eventually leading to the adopt of 5.56x45mm NATO), before changing into a West German program to create a new assault rifle that would increase "hit probability" or the ability of a soldier to hit the same target with two or more rounds over then-current assault rifles. Between the introduction of the first prototypes in 1974 and the eventual end of the project in 1993 with the reunification of Germany, the cost to the taxpayer was roughly equivalent to $92 billion (when adjusted for inflation) with H&K in almost twice as much debt. The cost of the program and the impossibility of recouping it due to NATO having standardized on 5.56x45mm NATO rather than the 4.73x33mm round the G11 used led the newly reunified Bundeswehr to declare "Nein!" in 1993. And while the 1000 rifles produced before the axe came down have been tested (formally and informally), there are no plans by any country to adopt it into service.

Amusingly, the march of time has seen that other technology that was the source of much fascination in 80s cyberpunk culture, namely advanced polymers and composite materials, actually succeeding where caseless ammunition has found difficult ground. The advantage that polymer-cased ammunition shares with caseless ammunition (reduced weight) is driving efforts by the Department of Defense and its allied partners to investigate adopting the technology in the near future. Meanwhile, caseless ammunition seems destined to remain limited to the R&D lab for years to come.

Yes, I started this as a short paragraph and then it got away from me. I regret nothing.

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CdrMike, Overwatch Reject

All the times I wondered about that name, and I never got around to researching it. Good to know.

Peter Eng
--
And not calling it a primer sent me looking into the pronunciation of that word.

Headspace is important because, as the name suggests, it determines where the head of the cartridge is when a round is loaded, and that is critical to safe functioning (or, in some cases, functioning at all). Too far forward, and the firing pin either won't hit the primer hard enough for reliable ignition or, in extreme cases, won't reach it at all. Not far enough, and the cartridge head protrudes from the chamber, which will hopefully prevent the firearm from going into battery at all, but in the worst case can lead to a situation where it fires out of battery, i.e., when the chamber is not fully closed. An out-of-battery ignition will do bad things, not limited to damaging the gun and causing bits of it and/or the ruptured cartridge case to fly back into the shooter's face.

Exactly what the stopping surface (what engineering draftstmen call the datum reference) is will vary from cartridge to cartridge, but they all have one that is defined somewhere in their type specification. Rimmed cartridges are the simplest; they virtually always headspace (it's also a verb) on the front of the rim. Most old-timey rifle and revolver cartridges are rimmed for exactly that reason. It's a simple, reliable way of doing it; if the rim of the cartridge case is the right thickness and the chamber(s) of the firearm is/are designed and made correctly, it'll always work. The chamber surface the rim rests against isn't subject to a great deal of wear in normal use.

Headspacing on the rim obviously doesn't work if the case doesn't have a protruding rim, though, and rimmed cartridges have their problems—they're extremely clunky to make work in mechanical feeding systems, for instance. For non-rimmed cartridges, some other solution has to be found. A lot of these have been tried over the years, but nowadays there are basically three different ways in which manufacturers headspace rimless cartridges:

- Belted cartridges are rimless at the head, but have a wider spot built into them ahead of the extractor groove—they're basically rimmed a little farther along. This is pretty rare nowadays, but used to be quite popular in certain high-powered rifle cartridges meant for the taking of big game. In a feed system, the belts can get hung up on each other just like cartridge rims do, but they're often not as pronounced as rims (and the rear surface can be angled), to try and minimize the likelihood. They're not usually used in anything that's going to be trying to feed automatically, anyway, which is where rimmed ammunition gives the most trouble.

- Cartridges with a shoulder (e.g., most modern rifle cartridges) will often headspace there, with the shoulder stopping against the angled "throat" of the chamber. This works reasonably well, but it does put the all-important bearing surface quite near where the most heat and pressure happens while firing, out where the bullet comes away from the case. As a result, rifles that headspace this way have to be checked to make sure their chamber throats haven't eroded out of tolerance. (My CMP M1 Garand, for instance, came with a tag certifying that its chamber throat was within acceptable tolerance as defined by the old Army standard.) Also, because the affected part is at the front of the chamber, in a place not easily seen, checking for those tolerances is a bit of a fiddle.

Armorers check the headspace in rifles like the M1 using a set of gauges, basically dummy cartridges that are precisely made to the maximum and minimum sizes of the rifle's specified headspace range. You load each gauge into the rifle and close the bolt (in the M1, by letting it close under spring tension, as if you were loading it normally). If it will close and lock on the minimum (commonly labeled "GO") gauge, but the maximum ("NOGO") gauge protrudes enough to keep the bolt from locking, you're good to go—that result means that the rifle's headspace is between the two values. Any other result is an indication that the headspace is out of spec and the rifle is unsafe to use.

(There's also a third gauge, usually called the "field" gauge, which is the absolute maximum safe length. Using that one when you don't have the other two handy or don't have the time to do both tests, hence the name, will not absolutely tell you the rifle is safe, but it will absolutely tell you if it isn't. If the rifle closes on the field gauge, the chamber is way the heck too long and it probably needs to be rebarreled. If it won't, it might be too short, but you're probably good to go, since in field conditions, i.e. with a rifle that has already been in service, excessive headspace is far more common than insufficient headspace.)

There are also some automatic weapons (e.g., certain WWII-era Soviet machine guns designed to use 7.62x54mmR ammunition) that use rimmed ammunition but headspace on the shoulder of the cartridge anyway. I'm not sure why, presumably it has something to do with the feeding system.

- Straight rimless cartridges, such as most pistol cartridges meant for use in automatic arms (e.g., 9mm Parabellum, .45 ACP), headspace on the front of the cartridge case, which is of slightly larger diameter than the bullet. (This is as opposed to many rifle and revolver cartridges, which are crimped at the base of the bullet so that there isn't a "step" there.) This is one of the reasons why a lot of semiautomatic pistols don't like reloaded ammunition; cases that have been fired and resized in a reloading die are usually a bit stretched, unless the reloader trims them back again, and so are no longer exactly the correct length.

As an aside, it's really kind of uncanny to me how perfect a material brass is for the job of a cartridge case. It's just strong enough to withstand the forces involved (if loaded correctly and used in a properly working firearm of the right type), but just ductile enough to do what it needs to do in terms of conforming to the chamber, providing an internal gas seal, and so forth. It absorbs a useful amount of heat, but not too much; it's dense enough to be sufficiently sturdy for field conditions without being so dense that ammunition is prohibitively heavy. On the face of it, the layman might think it was odd that alternative case materials have taken so long to get a foothold in the market (aluminum and steel are both becoming fairly popular for budget ammo now, though you can't reload it and IMO it's a bit shite, and there's the aforementioned experimentation with polymer cases going on nowadays), but really, brass is just such a Goldilocks material for the purpose that it's really not so surprising.

--G.
* Seriously, this is one of the great mysteries. The Maine Principals' Association, which keeps track of these things, uses something called the Heal Point System and it's like they're determining who gets into the basketball tournament using the Maxwell equation or something. High school athletes in Maine learn to do higher math in their heads just to keep track of whether they're in the playoffs.
-><-
Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
Ceterum censeo Carthaginem esse delendam.

First, what it is: "+P" is a bit of nomenclature used by the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), the US-based organization that maintains manufacturing standards for civilian small-arms ammunition. It indicates a variant cartridge loaded to a higher pressure than the baseline SAAMI standard for that cartridge. For instance, .38 Special +P has a rated chamber pressure of 18,500 pounds per square inch, as compared to "regular" .38 Special's rating of 17,000 psi. Extra pressure translates to extra velocity and thus greater muzzle energy (which is why you will sometimes see the "P" misrepresented as standing for "power").

The thing about +P is that, by design, it's not that much hotter than the baseline cartridge of the same name. There's a big gap between, say, the slightly hotter .38 Special +P and an actual .357 Magnum, which has a maximum rated pressure of 35,000 psi. (Fortunately, the .357 Magnum's designers thought of that, which is why they made the case—which is otherwise identical—too long to fit in .38 Special chambers.) By loading +P ammunition in a firearm that can handle it, you're getting a little more power, but you're not going to pierce the heavens.

The reason this is done at all is because, so the theory goes, modern metallurgy and manufacturing techniques make newer firearms in early-20th-century cartridges capable of withstanding higher pressures than were thought prudent when those cartridges were originally developed, and we might as well take advantage of some of that excess capacity. To that end, +P ammunition is designed so that if you fire it in an older gun that chambers the same cartridge, but isn't rated for the +P version of that cartridge, you won't blow it up and kill yourself. It's not a thing you ought to be doing, you'll wear out the gun much faster and eventually it will become unsafe, but it's not an instant ticket to Bad Times, like handloading a .38 Special case to .357 Magnum pressures and firing that would be. (People really do that. The technical term for these people is "bubba". And, in some cases, "Seven Fingers".)

The list of cartridges for which exist an actual, official SAAMI +P standard is very short. Off the top of my head I can only think of three: .38 Special, 9mm Parabellum, and .45 ACP. (There was one for .38 ACP, not to be confused with .380 ACP, but that eventually got renamed .38 Super, possibly because people kept confusing it with .380 ACP.) Which is an important point: if you see other things with "+P" on the box (the aforementioned .380 is a common culprit, because it's a popular defensive cartridge but often considered underpowered), that's at best the marketing people having you on, and at worst an unofficial hot load.

Similarly, if you ever see anything with "+P+" or the like on it, that is certainly unofficial, because SAAMI doesn't use that label. That's like old-timey porno producers claiming their products were rated XXX. There was no such thing, they just added the extra Xes to imply that their movies were super-hardcore.

Just a heads-up for informational purposes. You want to throw something you found labeled ".380 ACP +P+" in your '70s-vintage Mauser HSc, be my guest, it's your hand. Just let me leave the area before you shoot it.

--G.
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Benjamin D. Hutchins, Co-Founder, Editor-in-Chief, & Forum Mod
Eyrie Productions, Unlimited http://www.eyrie-productions.com/
zgryphon at that email service Google has
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