Interesting you should mention that. When I got my first AR... the 1:7 H-bar... I was pretty disappointed with the accuracy of generic 55grn FMJ. Even my handloads weren't that good. Jump forward 10 years and I shoot either 62grn M855 or my 69grn handloads... and things improve nicely. I don't know what to think about your comment that it doesn't matter... because at the end of the day, I don't have 2 equal 1:7 and 1:9 rifles to compare. I suppose someone somewhere has done that comparo... Was it just my crappy ammo, or does it really make a difference? Me thinks that if it realllly didn't matter, barrel makers and rifle builders wouldn't fool around with different twist barrels. Just my opinion.
Molon has done extensive testing. His findings are 55 gr bullets in a 1:7 twist barrel are not less precise than from a 1:9 barrel. Barrel twist does matter, but it's just a part of the whole. Barrel and ammo quality is just as important. Maybe more so.
Sierra did tests with doppler radar to measure the stability of their 69 gr Matchking fired from various twists. They found that the tighter the twist, the more stable the bullet which resulted in a better BC in flight. The 1:7 twist was the tightest they tested and resulted in the best stability.
I must point out that every barrel is a law unto itself. Some barrels defy "conventional wisdom". A shooter will never know how a barrel shoots a particular load until it's tried. The 1:9 twist is considered too slow to stabilize the longer bullets, generally meaning anything heavier than 62 grains. Yet, there are shooters who consistently shoot MOA or less using bullets in the 70s range with a 1:9 barrel. There are shooters who can't get anything less than patterns with 55 grain bullets from a 1:7. Both examples are the exception, however. My 1:7 ARs will keep 20 or 30 rounds inside 2 or 3 MOA using cheap blaster grade 55 gr bullets and a 4 MOA Aimpoint.
I try to stick with the 1:7 twist because I and others I shoot with, talk to and correspond with, find it to be generally more forgiving than the 1:9, especially with the reduced velocities of shorter barrels. As the 1:7 barrels I'm using work fine with 55 grain ammo, I see no reason to use a 1:9. I want to stack the odds in my favor. But that doesn't mean
you shouldn't try a 1:9. A 1:9 may work perfectly for you and if it works, it works and you win. If it doesn't work, you still win. You got out, shot the rifle and found out for yourself. In fact, you
did go out and shoot. That's far and away better than reading about it on the internet.
