Although I have a few rounds of XM855 and Ranger on hand, I now only buy American Eagle XM193. I believe this listing works out to .33 per round. I have seen it as low here as 26.99 per box of 90, and it is on stripper clips.
Federal 5.56 55gr FMJ-BT 90rds on Stripper Clips XM193AF90
I keep an M&P 15T mostly for the range. I prefer .300 Blackout if the stuff ever hits the fan. It's a far better round, and not NEAR as loud as 5.56. I have an Advanced Armament MPW in 3B, with a URX II rail. M&P 15T has the same rail covers, looks almost the same, and feels the same except for the trigger. Eventually I will upgrade to the same Geissele trigger in the M&P.