Well it’s a movie, so there’s that. Helmets primary use is to stop shrapnel and protect against bonking your head. I’ve heard stories of, and seen pictures of helmets stopping bullets, but there’s lots of factors. How far away did the bullet originate? Did it ricochet off something first? War is a very chaotic environment. Lots of weird things happen. There are stories from WWI and WWII where a bullet was stopped by a bible in a breast pocket, but it’s really up to random chance.

I believe most modern helmets are rated to stop a 9MM, but beyond that it’s a crap shoot.