Unless I missed something, the OP is engaging in a running and shooting contest, using a Garand. He isn't engaging in combat, or using a long gun with a magazine to monopod off of.
Looking at the rigs on those two guys in the picture, I think it is a clue they are standing next to a vehicle.
I don't know squat about combat. Besides the Keneyathalon, which seems close to what the OP is participating in, I have considerable experience shooting from slung, prone in the hunting field. I can tell you that things on your chest, whether they be large binoculars or handguns, are a detriment to good shooting with a sling.