People have figured this out; it's basically her visor, just about the entire thing and maybe then some.

It's actually probably way too big. If you've ever charged into combat as D.Va and noticed your health suddenly taking a giant nosedive despite the fact that you're a tank (and possibly being healed!) it's because everyone firing at you who is just hitting your enormous visor by pure dumb luck is getting headshot damage.

-Merc
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