I'd just build an upper in 6.8 SPC vs. the weight/cost of the large frames.

I have six large frame AR rifles 16, 18, and 24 all in .308... I don't take them coyote hunting as my 6.8 is a DRT rifle using 120gr SST bullets.

I guarantee you the Speer 70gr Semi-Spitzer will solve your run-off problems if you stay with .223.

Check this video out... three DRT'S