I am and have been frustrated with the "look" of firing automatic weapons in low/no-budget movies. Filmmakers who are much better than me insist on using blanks in their weapons. I hate blanks. Hate them almost to the degree that I would hate using live ammunition in a movie.
So the whole movie problem with the HK G36 is that the special magazine is transparent. I'm sure that's just awesome if you're, say, a soldier and you'd like to know how many bullets you have left.
But for film that just freakin' sucks. Because you can clearly see that real bullets are not cycling through the weapon.
But if you have five, or six, or seven bills to lay out for an airsoft version of one that will actually blow back the action, you can get one. Or, you know, you can find one that's cheaper.
Still, it has that darn plastic magazine with fake bullets in it where you can plainly see that the ammo isn't cycling.
So I'm looking at airsoft blowback guns. I know, I'm not the first person to think of this. But those guns can rock the action really nicely. I mean, just look at that M14.
Problem? Well they can be expensive.
The other major issue is that we nominally want guns that don't really look like regular 2013AD guns. We want something a bit more futuristic. Ironically the older M14 in all black (above) looks a tad more future than the AR15 variants we're so used to seeing (even though it's an older weapon).