I have a matched upper and lower (stripped) from Red River Tactical.  I want to do a build with it since they are local to me, and I think their logo is cool (hand grenade).  I have built up several AR's from stripped lowers, but have not assembled an upper.

Here's what I already have.

Armalite 16" midlength, M4 stock, post ban muzzle brake, EO Tech on RRA Dominator mount

Armalite M4gery built from a stripped Armalite lower and a "Christmas sale" Armalite upper, Weaver 1.5-4.5x in Larue mount

Sabre 18 inch stainless fluted upper with DD rail fore end on an LAR grizzly lower, Weaver 2.5-10x in Larue mount

Sabre 14.5 inch middy upper on a S&W lower, iron sights

50 Beowulf upper on a TNW billet lower with ACE skeleton stock, Leupold 1.5-4.5x in Larue mount

What should I build on the RR receivers?  I think I have enough 223/5.56 stuff, and the 6.8 doesn't really appeal to me that much. Considering a 458 SOCOM but not sure it will work on a standard upper receiver???

Looks like you've got the 223 barrel lengths and versions covered pretty well, maybe you're missing a long barreled varmint gun.OR you could do a 18"-20" 6.5 grendel.You've got a 50 Beo so doing a 458 is a waste to me.Or a Mark 12 clone, though one of yours is pretty close.How about some pics of the family to help?

22lr perhaps?

I neglected to mention that I have a S&W M&P .22

I've always been interested in the 7.62x39 in an AR, but this does not seem to be very popular at all.

aaaargh!

I got three little letters for you...........................................SBR

A Mk18 clone to be exact.