Yes you can. I've done it personally with a ma deuce on top of a HMMWV, and the ma deuce at my TC hatch on my M1A2 Abrams. Lol - you've never pulled a real trigger. Hehehehe

Yes you could. My brother is an veteran who was in the marine corps and now owns his own company training police forces swat teams and other people including individuals on how to operate weapons and situational control. In some cases I have used a full auto assault rifle. You can easily squeeze off one very precise round. However pulling and going full auto is incredibly difficult to maintain accuracy. So in the game running a clip of 50 rounds into a guys head is highly unlikely in real life.

With enough practice, you might be able to. Just a light trigger pull. A single round would be relatively accurate given that you aren't having to combat the constant muzzle climb. (Mind you, I am basing this off my limited [one afternoon] of getting to fire a couple sub-machine guns, where single shots were possible, and decently accurate). I think the aim-assist helps with this to an extent as well. And as others have said, realism was chucked out the window within the first minutes of the game, just roll with it. In the grand scheme, scoring single round headshot with a man-portable MG isn't /too/ farfetched. Hell, its the most realistic thing I've seen in game. You want to talk weird guns: look at where the mag is for a scout rifle: toward the back like a standard bull-pup design. But then some have cut outs where the mag well/feed would be. HOW DO MY SCOUT RIFLES GET BULLETS TO FIRE? This has bothered me since I saw it, lol.

I'm pretty sure guardians are modified considering the things they can do so that shouldn't be much of a problem for them