Dabs - If you do decide to go with a big bore upper, I would strongly suggest the .458 SOCOM.

The 450 Bushmaster is a good round, but it uses .452" bullets (same as a 45 ACP, therefore a pistol bullet) whereas the SOCOM uses true .458" diameter rifle bullets.

The Bushmaster requires a special magazine, but the SOCOM uses a standard 5.56/.223 AR mag with no modifications.

Yes, the Bushmaster is SAAMI approved and as of right now, the SOCOM is still pretty much considered a 'wildcat', but that will probably change this year, and when that happens, look for it to really take off.

Just something to think about if you get the EBR building bug.