Skip the 223. I have had 2 now as ranch rifles, I have always thought they were under performing rigs. 243 Win is great but eats the barrels, quick if your a volume shooter. I think the 6mmBR or 68 will be your best bet, lots of capible bullets in 6mm with exelent reach (70gr Nosler BT Hunting, 80grTTsx, or 80gr Partitions) but will be a bit tough being 20", the 6.8 has better energy and better bigger game bullets, will perform well with a short barrel but give up some range (400yd max).