[tahunua001]

it's not that every company makes a different kind of AR10 it's that DPMS had to change a few things to get around the armalite patents and most everyone else went with the DPMS design so they wouldn't have to pay royalties to armalite... or so I've been told, who knows if it's true. the big thing to remember is that the majority of parts available are only for DPMS pattern AR10s rather than armalite AR10s(DPMS has to call them LR308s legally).

ar15s are a completely different ballgame. unless you buy something that was cobbled together in someone's garage out of old beer cans and scrap steel then everything fits everything else, regardless of make. there are so many calibers available for AR15s it's not even funny but to name a few:
22lr
221 fireball
5.7x28FN
5.45x39mm
7.62x39mm
223 rem/5.56x45mm/223 wylde
300 whisper/AAC blackout
300 fireball
6mmTCU
6x45mm
6.5 grendel(next on my list if I ever decide to build another one)
6.8 SPC
7mmTCU
450 bushmaster
458 socom
50 beowulf
9mm luger
40 s&w
45ACP

and those are just the ones I can come up with off the top of my head. there are more than likely dozens more I forgot or have never heard of.

in most cases with ARs, you have two take down pins that hold the upper reciever assembly(which includes that barrel, handguard and bolt carrier group) to the lower reciever(which includes the trigger group, buffer group, pistol grip and buttstock). simply pull those two pins and strap an upper half to the lower receiver and you've effectively changed the caliber of the rifle. it would be a simple matter for me to change my 5.56mm AR15 to 5.7x28mm or even to 50 beowulf in a couple minutes assuming I had the upper halves for those laying around. not all calibers are that simple, pistol calibers like 9mm require some changes to the buffer, hammer and magazine well so it's best to have a dedicated rifle for calibers like those.