We used to just strip a couple of clips or pick up a slack handful of ammo in our right hand (or left hand if you were one of the cack-handers), hold the mag on our left knee and slip the rounds in, push back. next in, push back next in push back and so on. After about a minute it became second nature. I do remember that the gun crews REALLY used to get arsey if they were shouting for magazines - and you passed them part empty mags. Watch the foul language fly then.............