A hint to keep the sling from slipping:
Firm Grip (spray adhesive), spray it on the rubber pad on your shooting coat. Also spray some on the rifle where your gloved had goes.
You shoot (in rapid fire) two 10 shot strings normally. Come out of position between strings and re-apply the sling. The same with slow fire. After about 10 rounds (of the 20 round string) come out of position again, redo the sling.
Also it wouldn't hurt to put firm grip on the butt of the rifle, in off hand, on your left elbow (if right handed) where it come in contact with the rib cage.
Spray some on the shooting mat where your elbows go in the prone, also it wouldn't hurt to put some on the mat where your knees line up. I would recommend some shooting pants with rubber pads on the seat, and knees.
On the M14/M1A, here is an example of where to put the Firm Grip.