I know this thread's a leetle old, but wat the hell
I would only recommend doing this to protect your fingers for game terms. Otherwise, yes, it's a little unrealistic. Real assault weapons would turn the wee-shield into a shrapnel concern. Ensure it doesn't block vital areas of the gun, make mock ups and try gun playing around corners and in prone. Think of the hand guards on old swords. Start in a large size, and shave down areas that are unnecessary. I would venture you use plywood and scroll saw the profile of the gun into it. After you've tested it and are happy with the shape, staple camo fabric or a fake leather over it.
Or just wear hockey gloves, :p