No experience with it in an AR, but have an ancient 300 Whisper barrel on a contender some where... It keyholes at 40 feet. It was from Fox River (TC's custom shop) and it has too slow of a twist. I did contact JD Jones about it, and he laughed and said "Let me guess...Fox River." He offered to rechamber it to one of his stouter cartridges, and I never did send it in. As for AR's, the only thing I've seen (besides a fast twist for sub-sonics) is keep it FAR FAR AWAY from your other AR's (223/5.56) at the range. I've seen more than one "kaboom" photo from someone unknowingly inserting a 300BLK magazine in a 223 and then chambering a round, c l i c k. Complete destruction ensues.
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