After much thinking, the AR10 platform might be better cost wise. 308, 243, 260Rem, 7mm08, and 338 Federal, but the 358 wouldn't be hard either. All use the 308 case, no bolt or bolt carrier to change out. The 308 by itself isn't a slouch either. Everything from plinking 30 Cal carbine bullets, to 200 grain heavies. 220 is probably a bit much for the 308. Most everything can be hunted from the standard 110 gr to the 200 grain. From varmits to bears.
However I also think the 6.5 Grendal is probably the most efficient powerwise of and of the AR-15 rounds.
