I have a oly arms .45 upper with a cav arms lower and the hahn block. It uses grease gun mags and it is a boat load of fun to shoot. Its a shame that it costs so much for .45acp nowadays. Also grease gun mags are like $25 a pop now. The reason why you need the cav arms lower is because the grease gun mag is too wide for a standard one...just in case you were wondering.