Barrel Rifling Twist Rates Explained
BY Herschel Smith
1 year, 9 months ago
Ideal twist rates produce a gyroscopic factor between 1.5 and 2.0.
Factors between 1.0 and 1.3 are marginally stable, but they’re generally considered too slow. Factors between 2.1 and 2.9 are fast, but stable and accurate. Factors above 3.0 are suitable, but not ideal. Climbing above 4.0 may cause over-stabilization of the round being fired, which can harm accuracy. The optimal twist rates for 5.56 and .223 loads are:
- 45-gr Varminter: 1:12 twist
- 55-gr (M193): 1:9 twist
- 62-gr (M855): 1:8 twist
- 77-gr (Mk262): 1:8 twist
- 80-gr Sierra Match: 1:7 twist
- 90-gr Sierra Match: 1:7 twist
The way I read the table, 1:9 twist rate is good for just about anything up to 77 grains. I wouldn’t shoot anything above that in 5.56mm anyway. Heavier bullets than that need to be .224 Valkyrie, 6mm ARC or 6.5 Grendel. I once thought that .224 Valkyrie was a flash in the pan, but occasionally I do see it at Academy and Cabella’s. It’s also possible to pick it up via Ammoseek.
I don’t have anything in that caliber and would not. I like the 6mm ARC too much to switch to something less effective and versatile.
On March 20, 2023 at 6:43 am, Joe Blow said:
The best discussion I’ve come across was a video series by a guy “tibosaurus rex” or something on Oy!tube. He got into rifle harmonics, bedding, scope mount position, etc…
Short version: shoot everything you can until you find a round your rifle ‘likes’, that may require handloading. After ~20 some boxes, my AR likes the Wolf 62 grain… boxes and boxes on top of boxes, thats all I buy. 6″ groups at 200 yds works for me.
On March 20, 2023 at 3:10 pm, Drake said:
Last month I took my 1:8 18″ (5R) AR to the range with a variety of ammo to see what groups I got at 100 yards.
55 grain was around 3″
62 grain a bit under 2″
Norma 69 grain Golden Target was .5″
AAC 77 grain .8″
AAC 75 grain .7″
It was a cold day and I’m not swearing by my marksmanship – but it really seemed to like the heavier bullets. Since the AAC 75 grain stuff was by far the cheapest, I went out a bought it in quantity.
On March 20, 2023 at 5:25 pm, ZERO[F2G] said:
This is a topic I have always enjoyed because twist rate does factor, but there are so many other factors at play as well.
I have AR-15’s in multiple calibers, but only two are chambered 5.56.
One is a 20″ A2 patterned thus not free floated. It is a heavy barrel and chrome lined(which as I understand can effect accuracy a bit, but longevity is gained) twist is 1:9.
It shoots 1″ groups with 55 and 62 quality ammo, and groups open up to 1.5 to 2″ with some of the cheap cheap 55 and 62 grain Russian stuff. I have not shot any heavier or match grade ammo through it, although I would like too and hopefully will this spring.
The other 5.56 is a 14.5 colt barrel standard profile also chrome lined and free floated.
It shoots 1.5 to 2″ groups both with 55 and 62 grain good ammo I really don’t see a difference, ammo like tula and wolf tend to open up a bit to more like occasional 1.5 groups but more often 2 to 3″ at 100′.
I have shot very little heavier bullets do to price, I did shoot several groups last spring with some 77 grain Privi Partisan. I was able to shoot a couple of groups at .75, a few were around 1″ and a few were around 1.25 before exhausting the 40 rounds I had.
My conclusion was that this being a defensive rifle that I don’t foresee needing beyond 300′ capabilities and more likely much less. 55 grain loads offers enough accuracy and and the cost point means more training rounds fired and more rounds available vs. the cost of heavier more accurate loads.
Joe above is referring to a fabulous 100 plus video tutorial by Rex that really dives into a lot of the science from A to B, it is no longer on Jew tube one has to subscribe to Rex’s patrion account to gin access.
But yeah, he goes over the importance of twist rate, along with the dozens of other factors involved.
On March 20, 2023 at 6:42 pm, Herschel Smith said:
Yes, I’ve seen Rex’s videos.
On March 20, 2023 at 7:48 pm, Rocketguy said:
I just worked up a load for my 1:7 20″ 5.56….40 grn V-Max at almost 3700 fps. It’s not *supposed* to work but it almost stacks them on top of each other. Probably not far from vaporizing them at the muzzle. Can’t wait to see what the groundhogs think.