Assuming that the barrel length is the same 20" as on a standard AR service rifle...
1:9 shoots up to 69gr HPBT bullets very well, some can shoot 75gr HPBT well
1:8 shoots 80gr HPBT for single load stages, usually 77gr HPBT for "across the course"
1:7 the "milspec twist" shoots 80gr HPBT well, handles 77gr HPBT no problem, but isn't fast enough for 90gr HPBT
Anything 1:8 or tighter is fine for High Power matches. 1:9 is doable, but I only know one person who has done it (and he shoots High Master scores with 69gr SMKs) but is also a .mil sniper in his day job.
If the barrel length changes, then all bets are off. A lot of competitors like a 1:7.8 twist because it stabilizes the 80 grainers a tad better than the 1:8 twist without introducing "bullet imbalance" on the target like a 1:7 can. For the same bullet a 1:8 twist will introduce almost a tenth of an inch less variation on target due to bullet imbalance.
If you have a different barrel length or cartridge combo in mind, let us know.
Jimro
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Machine guns are awesome until you have to carry one.
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