If the body is not sitting flat against the rail, then the bolt spring and/or bolt lip can rub against the body. This could happen if the pim bottoms out in the rail.
Except for the Automag in front, its usually the man behind the equipment that counts.
The pim is the mounting post on the bottom of the body.
Post more pics of the rest of the gun.
"Accuracy by aiming."
Definitely not on the A-Team.