I don't know if you're still working on this "piece", or if you've decided to do something else, but I've got a few pointers.
First of all, if you have any experience with shooting, rifles, pistols or otherwise, you would know that you almost automatically find a stance that keeps you "on target", and keeps your hands steady. A rifle is heavy (yes, even a P90 sucks after an hour), so you want it to "rest" on your shoulder, even when you are not firing. That way, you can easily reassume your shooting position. Try it out yourself, imagine where you would place it. And then go by that. Take a look
here to actually see what I'm trying to explain.
Secondly, you will want to put most of the weight on you back foot, place your other shoulder a bit lower, and keep your head close by (but not touching) the rifle. Most people don't run and shoot, they tend to do one or the other. In real life that is. And if you have started shooting standing still, you usually pause your fire and start running, you rather seldom start running when firing.
What I've seen here so far, both original and edits (except for KilledBySuicide's, that is pretty much straight to the point) would probably put an average Joe's bullets in the roof instead of the enemies, posewise that is.
So, if you want to do an accurate piece, I suggest you start by getting the anatomy and pose down correctly. And it's ok to go larger, you can always shrink it down and/or redo it later.