[Sgt KIA]

The mod is called rifles on back or something like that. AFAIK its done by scripting. You can edit holstering animation and bone positions and set the weapon not to disappear after the animation is over. More info at TMT.

But the one Wilkins referred to I made by editing the pouch and the rifle. Its a separate model and the rifle stays on your back even if you use it as your primary weapon.
Wilkins see your pm.