So...if you don't mind entertaining this one...does a significant difference exist that would see the 55gr performance acceptable through the 1:9 setups where the 1:7 were unacceptable? [same platform] or does the 62 + still apply?
It seems that the minor variance in twist rate would have near insignificant performance change...that's why I ask.
Does the bullet composition factor in...as in JHP -vs- FMJ -vs- FMJ Penetrator...etc?
-[To complete the picture...consider it to be a bushmaster M4A3 w/ permanent Izzy ...a 14.5 barrel w/ fixed flash supp to total 16" length, 1:9 twist (6 grooves and lands)]-
Well, the 1:9 twistrate was Eugene Stoner's idea, and it performs well with BOTH 55s and 62s. However, it understabilizes somewhere past 70gr and is not very effective with the 77gr match rounds at range (beyond 400-ish)
Twistrate is mostly about which grainweights the barrel can handle at the expected muzzle velocities without incurring dynamic stabilization errors. You would assume a given twistrate and then have a range of projectile weights it can handle.
1:12 goes from something like 35gr up to 57gr
1:10 I think is 45gr to 68gr
1:9 I think it 50gr to 71g
1:7 I am confident is 61gr to 80gr
So, when it comes to picking projectiles for barrel twistrates, just keep in mind the above chart, knowing that 1) if the round is too heavy, it will understabilize, if the round is too light it will overstabilize, and if the round is WAY too light, it will shred.
I could get into a litany of just WHY understabilization starts having accuracy problems immediately that increase, while overstabilization starts having problems when the projectile is heading "downward" or beyond 300 -ish yards. But that is another topic for another post.
With a 1:9 you can use (I think) as low as a 50gr (leave the 45s for the 1:12 barrels) I'd push up to a max of 69gr to 70gr unless "everything" was within 300 yards, at which point it hardly matters.
A 1:9 is the most "general" twistrate available for this caliber. Well, some would argue the 1:10 is better for "all around many rounds" because it will handle from 45gr to 68gr but I don't have the background to argue there at all.
But pretty much ANYTHING you can physically fit into the mag of a 1:9 weapon, you don't have to worry about shredding or overstabilization, or even understabilization unless you go with a really heavy round, and that only starts effecting the round somewhere between 250 and 300 yards.
i think the reason there is so much effect with minor adjustments to round weight and twistrate is because these projectiles are being fired at extreme velocities near the outer edge of their performance window.
Also, on the surface of it, a 7 grain increase from 55 to 62 doesn't sound like much until you realize that the round is awfully light to begin with. That's something like a 13% increase in projectile size.
I like having a 1:7 twistrate because i know if i had to get come fancy-dancy 85gr hand-loaders that I could put them on target....even with an AR...at 850 yards without issue. (except maybe the wind...lol)
The sort of round matters a lot less than the weight and twistrate. I have not studied the gyroscopic stabilization effect enough to tell you WHY there is such a tight relationship between round size, velocity, and twistrate, but the information I have collected is from the experts who do study it. About the only place the type of projectie would matter is if you were seriously underweight, say, a 55gr on a 1:7 barrel, a hollowpoint with a thinner jacket is more likely to shred earlier than a FMJ with a thicker jacket.
SO, if all the factors were to line up: you have a 1:7 twistrate barrel, you are firing 55gr hollowpoints with a thin jacket chances are your projectiles are high velocity copper and lead dust within about 50 yards as the centrifugal force literally shreds the projectile.
Thanks youngbuck! Thanks Gunny!
