Sorry, my bad, musta missed the post where the OP was specific about what he was after IE milspec M4. I just went back up and it must be deleted I guess.
Other than that, a 7 twist won't stabilize all bullets. A 9 twist thats true, will stabilize everything up to an 80.
I"ve shot a LOT of 5.56 practice stuff in 223 chambers and never had an issue.
Barrel quality I covered.
Carriers are an issue granted, but port size covers that issue, you are adapted to both carrier weight and bullet choice IE weight when you deal with either.
Non M4 don't have M4 cuts.
Yep 2 different things, but then again I"d take any of my match guns as 200% reliable to feed and function till they die any day. At least mine have run around 10K plus rounds give or take each year for many years, and we have 4 guns in the family most years, some years more like 6-8 with other juniors....
