so im not new to the handling of m16,ar 15's. ive been around them since 04, even slept with one. so a question i keep finding myself asking is about these black rifles is...i know they can be chambered in many calibers, .223, 6.5, .308, .458. how does this work. does the lower mag well have to be larger as well to accomodate the larger rounds? or does it turn into a one single shot rifle? so in theory could i use any lower and any upper chambered in different calibers? or does the lower need to be the correct caliber match?