After my Glock Mags have been loaded for a long time, i want to be sure they will function when i need them. Because the Mags spring will be it's weakest point with the last two rounds held i know if they don't feed live rounds there is a problem. I don't want to use live Ammo. in these tests so i use Snap Caps, i put two Snap Caps in the Mag. and rack them through, if the feed correctly i feel the Mag. springs are Ok. This is the best way i have found besides just shooting live Ammo.