Explain Barrel Twist Rate, 1/7 1/8 Etc... What Does It All Mean?

[JJTwat]

Can anyone explain barrel twist rate to me and what it means for the performance of a gun? What type or weight of ammo should be shot out of each one. I've been around for a little bit, but never paid much attention to these sort of scientific things. However I plan on doing my first competition shooting this year so I want to make sure I can get the most out of my rifle(s).


One barrel has a twist rate of 1/7, and the other 1/8. Further more how can I tell the twist rate of a barrel if I do not know it? Is it stamped on barrels some place?


Thanks for helping an old newbie out with what is probably a basic question regarding barrel twist rates.

 

[RangerTim]

Barrel twist refers to how many inches it takes for the lands & grooves (rifling) to complete a full rotation. The speed of this twist directly affects the stability of the bullet in flight. Much like a football. If it's rotating too slow you get a sloppy spiral and unstable ball. If it's rotating too fast it can also have negative affect; on a bullet I've actually seen the jacket separate from the core from hyper rotation (40gr bullet through 1:7 twist barrel). Generally, a barrel twist will be "rated" for a specific bullet weight range. IE - 1:7 will usually stabilize 55gr - 77gr. However, that can be gun specific. I've also seen two identical 1:7 Colt AR's where 1 shot 55gr just fine and the other wouldn't group for crap.


Most civilian rifles (AR's anyway) used a 1:9 as it did an adequate job stabilizing 55gr, 62gr, and even into the 70gr bullets. However recent trends are more towards the 1:8 twist. Mil-Spec is 1:7 twist as issue ammo is the M855 round with the 62gr SS109 bullet. Bullet accuracy can also be affected by the bearing surface (this is the amount of surface area on the bullet that is in contact with the rifling). Heavier bullets are generally longer and therefore have a larger bearing surface. While most of my experience is in AR-15 rifles, twist rate will have the same affect in any rifle. You'll find that varmint rifles in .223 may be designed for something like a 40gr V-Max type ammo and have upwards of a 1:10 or 1:12 twist. I own a Savage 24F .223/12ga and my .223 barrel is a 1:14 twist. I MUST shoot bullets in the 40gr-50gr range to get decent accuracy.


Summary:


Heavier Bullet = Faster twist rate


Lighter Bullet = Slower twist rate


Twist rate is almost ALWAYS stamped on the barrel in one spot or another


I only dabble in rifle. @Shawn Agne is the expert rifle guy on the forum that I know of that I'm sure can give you way more information. Good luck and I hope this helped!

 

[Dirtbike]

A couple of charts that I've been looking at at least add a visual to it, there are many many more, I'm not a comp shooter but these have helped me out a little and as Chris said; Shawn from the bit we've chatted here on the boards is Damn knowledgeable! View attachment 428 View attachment 429 View attachment 430

 

[Shawn Agne]

Often times twist is associated to bullet weight but in reality the longer the bullet the faster the twist you need. Best example is the 185gr Berger Hybrid vs. the 185gr Juggernaught. Both are the same weight but since the hybrid is longer you need a slightly faster twist.


When a bullet leaves a barrel it needs a certain amount of gyroscopic stability to be stable and the forces pushing on the bullet will be the greatest when the bullet leaves the muzzle and it lessons as the bullet flies farther. The Gyroscopic stability is referred to as Sg 1.0 is stable but a value of 1.5 or greater maximizes bullet BC. Your talking small percentages but if your shooting at longer ranges (600 or farther) every bit helps. You'll see the charts above but I'm not sure where they come from as a 1:9 twist will stabilize a 75/77gr just fine. You actually have to go to a 1:10 before you see these bullets not being stabilized. However typically you will see 1:8 and 1:7's. The 1:7 is the military twist and it is overkill even for the SS109 bullet, that twist is there for freezing cold conditions at sea-level a 1:8 will stabilize anything you will shoot out to 600yds and IMHO is the best way to go. The only 1:7 I run is in my 223 Palma rifle and I did that so I could stabilize the 90s to use at 1000yd.


As far as the little bullets in high twist barrels I don't think it is the twist that necessarily separates the jackets off rather the friction in the bore that degrades the jacket and thus causes the separation. If you reduce the friction (i.e. molly coating) the bullet will stay together. You see this with the .243 WIn, with that cartridge you can launch a 105gr bullet at 3,300fps out of a 32" barrel. When John Whidden was doing his initial testing with that cartridge he found that he needed to use molly to keep the barrel friction down. Also he discovered he needed to use a barrel with shallow lands so it didn't stress the jacket as much. Either of these to factors are enough to cause jacket failure which was common with the original Berger 105gr VLD which had a thin jacket if you didn't molly or shot a barrel with deep lands. Most varmint bullets are of similar construction with a thin jacket, you are talking about similar velocities so the above variables with the 243 will apply. This is probably why the 40's shread apart in a 1:7. If you would molly and if the barrel doesn't have deep lands it will work just fine. Any time you have a bullet failure the jacket has been perforated in some manner. I have a load I worked up for a 26" 1:12 barrel where 50's were pushing at around 3,420fps. I'm going to put that load in my 32" 1:7 twist barrel on my 223 palma rifle to test. The RPM values will be a lot higher but I think being molly coated and with the lands not being terrible deep the bullet will work just fine.


Does this answer your question?

 

[RangerTim]

Like I said...the expert!

 

[David]

Does barrel twist rate only apply to longer guns? I've never really seen it advertised for a Glock, or anything of the sorts, but I do see it advertised on AR15's and other guns with longer barrels.

 

[Shawn Agne]

Simply put with pistols it isn't a concern as the bullets are short and fat, thus don't need a lot of twist. Rifles yes it makes a difference as the bullets are longer. The only reason it is an issue is because the manufacturers insist on putting slow twist barrels on their rifles. Remington is notorious for this. The 6mm Rem had a slower twist barrel on than the .243 Win. Thus a factory .243 could shoot the heavier bullets and the 6mm which actually was probably the better cartridge couldn't and it faded away. Fast forward they did the same thing with the .260 Rem also. If a 1:8 would just be the industry standard for a .223 then there wouldn't be any issue for the majority of shooters.