Originally Posted by JonM
Biggest it gets and still be semi auto on a standard ar15 format is 458socom which at its best hits the extreme low end of 45-70 in power.
Dont forget the .50 beowolf either , the .458 socom has alot better choices for ammo though.
if you really wanted to get crazy you could always buy a .50 bmg single shot bolt action upper to put on your AR lower . then you wouldnt have to take it to have it processed it would be hamburger after the shot