[B.T.]

I'm not quite shure on that. I am actually torn between 3 weapons. The question was, what would be the weapon of choice for my PC!

1. The weapon should be compatible with existing supply chains.

2. The ergonomics of the rifle should work.

3. You'd better be familiar with the weapon or at least the weapon family.

My problem is a relatively short thump. I have never used a real life M16/AR15/M4, but I own a softair M4. The fire selector works like a real one. Because of my short thumb, I have really problems to set the safety lever to full automatic. In a firefight that would be a sincere drawback!

I'm not fond of bullpub designs. I was trained with a conventional design and therefore I'd neglect a bullpub.

So: If my character was supposed to be of Western origin (and still belonging to a Western army), here are the weapons:

FN FNC

H&K G41

M16/M4

All three can take the STANAG mags. I'd prefer the FN or H&K because of the (to my point of view!) better fitting ergonomics.
I know, that the supply chain argument is not really a strong one, but I suppose, that most soldiers in a unit would use rifles with STANAG magazines and therefore you could use mags of your comrades (and of wounded or fallen).

If my PC was to be part of the US armed forces and a born American, he'd supposingly use the M16.

Well, that's what I think.