Oh and yes there are already setups so you can change the upper on an AR and have a different caliber. I believe 6.5 Grendel and 6.8 SPC can even use 5.56 magazines with a different follower/spring. Then there are magazine blocks that can be used on a 5.56 lower so you can switch to a 9mm upper and use 9mm mags in the same lower.
you have to change teh hammer also & buffer
