slot, female, in the side of the receiver. An aluminum mount's Male "t" will SOON be worn to worthlessness if you remove, replace it much at ALL. The SLIGHTES amount of movent in a scope mount renders it a worthless pos. a STEEL M1A mount is one lb, SO is a scope that offers you ANYTHING in the way of greater range than the 223 is capable of, on men. 9 lb rifle, 2 lbs of scope and mount, a lb of bipod and sling, 20 rds of 308 150 gr ammo-one lbs. Steel M14 mags weigh 7 ozs EACH. I aint wrong when I call 308 rifles pointless old CLUNKS.