Check out this instructional weapon tutorial video to learn how to make an M16/M4 carving magazine pull-carry loop.
The M16/M4 assault rifle/carbine and family of 5.56mm weapons rely on 30-round magazines to feed ammunition into them for firing. During a firefight you cannot afford to waste seconds fumbling for a magazine in your pouch, nor can you stop and pick up an empty one if it falls out of an open BDU/ACU trouser pocket while firing/moving.
The 75th Ranger Regiment has its Rangers place a 5-inch piece of gutted 550 cord (remove the 7 white inner strands) with an overhand knot on each end onto the bottom of their 30-round magazines (See video). You simply remove the bottom plate and slide it carefully back with the two knotted ends tucked underneath at about the middle the knots under the last stretch of the magazine spring so the gutted 550 cord is flat so the bottom plate can slide past them. Once slid back in place, you now have a "pull-carry loop" to extract your magazines from your magazine pouch or Enhanced Tactical Load Bearing Vest (ETLBV) while firing and moving without taking your eyes off the enemy. Use black electrical tape or green "100 mph" tape to wrap around the magazine bottom to silence its metal if they come into contact with rocks etc. If you are in a hurry and cannot remove the magazine bottom plates, you can knot the ends of the 550 cord and tape them to the outside of magazines to form pull loops.
When you are done with empty magazines you slide their pull-carry loops onto a snap link/carabiner clipped to a LBE/ETLBV shoulder strap or magazine pouch grenade holder to retrieve them without fear of them falling out of BDU pockets.