Originally, no, it was just a chance per space the ammo took up. The problem was that sometimes you had that spare ton, and, well, you know, not close enough to the SRM-5, but right next to the MG, and you know, might see Elementals on the mission, so hell, we'll put some more ammo in there. And sure enough, that was when it went to Hell.

(Amusingly, there were other cook-off rules from ammo explosions. Once saw an Atlas taken out completely when a hit on the missle ammo spread to the AC/20 cartridge and that boy got split in half.)

--
Geoff Depew - Darth Mephron
Haberdasher to Androids, Dark Lord of Sith Tech Support.
"And Remember! Google is your Friend!!"