WHERE TO BUY AMMUNITION FOR TARGET SHOOTING NO FURTHER A MYSTERY

MoCo Purchased a rifle that takes AICS mags. It came w/ one PMAG10 AC. Thats very extended if applying susceptible (TIL AICS mags are one stack.) I need to obtain a number of 5rd versions. Looking online I notice magpul has new 'Sig Cross' versions w/ a follower that has a little bit feed lip in the front that will help if loading singles throughou