I took a look around for a thread about this, but I didn't find anything. So if I'm necroing a well known bug, I apologise.
At any rate, when I use the PSG-1 in the main game, it bugs out with my characters left arm. He's holding the gun much farther forward than he can reach, it seems, and his arm will stiffen unnaturally, and then tends to bend the opposite way an elbow should bend. It's pretty funny to see happen.