Twist rates- Faster is better!

[6.5 GUY]

I've been thinking of building a fast twist 1:9; 1:8 300 win. mag. or 30-338 FBI with a long throat for the 208-240 gr. bullets would give any big 338 a serious run for the money, and save a lot coin as well, and with less recoil. Those heavy 30 cal. bullets are now approaching what was previously only limited to the big 338's. Berger's new 230 hybrid has a BC of .743, and their tactical bullet has a BC of .714.

The idea of really fast twist rates is not really new, but is now taken more seriously by some rifle builders. Why spend the money on better bullets while running a slow twist barrel? These fast twist rates will pay big dividends down range. The more twist a barrel imparts more spin on the bullet, the more gyroscopic stability is retained through supersonic and into subsonic flight.

What is lost in subsonic flight is the loss of BC due to the loss of gyroscopic stability, which allows more oscillation, which is basically more drag, so BC is effected. If we spin the bullet faster to fight off the effects of transonic shock waves, the bullet will retain more gyroscopic stability which allows the bullet to retain more of its BC further down range. Resulting in better groups with less holdovers at distance.

I might get a bit of silence or a chuckle from a builder about wanting to use such a fast twist rate in a 30 cal. but it has been proven to work very well in testing. Even the big 338's are now going as fast as 7.8:1 twists.

I suppose though you'll need to know how much of a payload you want downrange hitting a target though? Animal, steel or paper targets? Weight to carry around and the recoil you can handle well enough to shoot well that is repeatable. Or should I just stick with my 6.5/300 WSM and leave the 300 Jarrett alone?

 

[rw496]

Are there any advantages to a slower twist rate?

 

[idaho1]

If I'm not mistaken, Todd Hodnet of Accuracy First did some work on fast twist, short barrel 300 wm and 300 Norma. He was happy with the results.

 

[varmint slayer]

Bryan Litz has a big write up on this very topic in his book. Read that and you will likely change your mind.

 

[Masked]

Have to say...I'm very happy with the 220 sierras at 1/10...

 

[BuzzBoss915]

I have not read what Brian wrote but it's fairly obvious that there is no free lunch. A fast twist is great for long range banging away at iron, but most of those bullets are not suitable for game, and being slightly cheapskate, I dislike rebarreling every couple years because I was/am driving heavy long bullets in a fast twist barrel. In other words, now you have a dedicated rig that won't handle short light bullets, without blowing up, but if that's your game and your wallet can handle it, more power to you. I still kinda like versatility. I have been guilty, as much as anybody, but: Hey, everybody has a dream. BB

 

[Jhnmdahl]

Are there any advantages to a slower twist rate?

Most people use only enough twist to properly stabilize the most unstable bullet they anticipate shooting in a given rifle because excess twist (i.e., above a gyroscopic stability factor around 1.4-2.0) can result in a less stable bullet at the muzzle and through the first couple hundred yards due to imperfections in shape or balance. This is becoming less of a problem these days as bullet quality continues to improve.

Note that a bullet traveling downrange loses velocity much faster than it loses spin, and so becomes increasingly gyroscopically stable as it travels downrange, often by several times. Dynamic stability problems, such as with the SMK 168 grain with the excessively large boat tail angle, are a bullet design problem, and can't be designed away simply by spinning a bad bullet design faster.

John

 

[Pinkycatcher]

Are there any advantages to a slower twist rate?

Increased barrel wear, think of it as pushing something up a steeper slope it requires more energy to engrave and twist. Also decreased theoretical accuracy (Lilja Precision Rifle Barrels - Articles: A Look at Bullet Imbalance and Twist) as the faster you spin something out of balance (which every bullet is even a little bit) the larger spread you get. That's more of a benchrest thing. Also you in theory lose speed.

For most purposes you're not going to spin a bullet so fast it vaporizes, especially in a .30 cal, you can kind of do it .22 and down.

 

[6.5 GUY]

If I'm not mistaken, Todd Hodnet of Accuracy First did some work on fast twist, short barrel 300 wm and 300 Norma. He was happy with the results.

And in fact did, including using a 1:7.8 for his .308. He had a prototype .308 rifle that had a 14" barrel and an 1:8 twist. it had no problem hitting a target multiple times at a mile w/the 168 grain ammo. The other rifles had to switch to using the 175 grain ammo to hit that far.
He also feels the 338 lapua should be using a 1:8 twist for the bullets out there.

Todd's 1:7.8 reportedly had 14,000 rounds before it shot out. He also had nearly 4,000 rounds on two 338 LM rifles that were still 1/2" @ 100m and wasn't seeing excessive wear. These could in theory also produce excellent accuracy at extreme ranges at reduced velocities which would extend barrel life.

 

[HodgdonExtreme]

It was my understanding that "super stabilizing" (my untechnical term) would result in an overly rigid spin axis, and the bullet would tend to remain in a slightly "nose up" attitude on the downward leg of the trajectory, thus increasing drag.

I'm no ballistic expert though.

 

[M40_A1]

Dynamic stability problems, such as with the SMK 168 grain with the excessively large boat tail angle, are a bullet design problem, and can't be designed away simply by spinning a bad bullet design faster.

John

IMO, the 168 is an excellent bullet, and only becomes a "design problem" once it enters the transonic region. They are usually very accurate within about 800-900 yards.

 

[Niles Coyote]

Bryan Litz has a big write up on this very topic in his book. Read that and you will likely change your mind.

^^this^^ and having a 1:8 308 and a few 1:7 223 I can tell you its not the next leap forward in my experience. The 223 does well with bullets that call for it but the lighter weights don’t group nearly as tight. On the 308 I have yet to find anything that will shoot consistently well. One trick that has helped came from Frank Green of Bartlien (spl) barrels and that is you MUST watch your runout when shooting faster than needed twists. Doing that and using the thickest necked brass to more closely align the bullet in the chamber shrunk my groups and got rid of the crazy fliers I was experiencing but I am still not shooting as well as I know I am capable of. What’s the longest bullet you wish to shoot from a 300? If it’s the berger 230 they say (berger) a 1:10 will work, so if it were me I’d go no faster than 1:9.5...

But feel free to experiment.

It may just be that my barrel isn’t perfect or as Todd Hodnet put it in his article that I read, paraphrasing here, “it hasn’t come into its own” yet as his did after 200 rounds.

FWIW, the 308 barrel I have will shoot moa now and occasionally group a few near the .7’s. What gives me pause is if I screw that barrel off and screw on a 1:10 or 1:12 I am half that with occasionals in the .3’s, be it at 100y or out long... if you factor out my horizontal dispersion from shooter induced screw ups on wind calls. I see this as well with the 223 1:7’s when I go from 69-80 grain bullets down to the low 50-53 grain weights, group size just about doubles.

 

[Jhnmdahl]

IMO, the 168 is an excellent bullet, and only becomes a "design problem" once it enters the transonic region. They are usually very accurate within about 800-900 yards.

Yes, I agree completely. Dynamic stability problems are really exacerbated in the bullet's transonic region, and the SMK 168 is still a beloved and accurate bullet for distances under 800-900 yards.

 

[KYpatriot]

A bullet is either gyroscopically stable or not when it exits the barrel. If it is stable on exit, then it will actually get even more stable as the forward velocity slows and the overturning moment decreases, because the forward velocity decays faster than the rotational velocity. "Over stabilizing" with more twist than is needed is going to hurt rather than help in most cases, because most of our targets are engaged when the bullet is supersonic and any more spin than we need just accentuates any imbalance in the bullet resulting in degraded accuracy. This is the reason bench rest shooters choose barrels that barely have enough twist.

Besides, lack of gyroscopic stability is not the reason some bullets fail to transition to subsonic in a repeatable predictable way, it is more complicated than that. Subsonic bullets actually have far less drag on them than supersonic bullets, and loose energy at a much lesser rate than supersonic bullets. A successful ( "predictable" ) transition to subsonic has to do with more than rpm on the bullet and is affected by the shape, center of pressure, etc. as the supersonic shock waves move forward as the bullet slows and then cease to exist altogether, the center of pressure moves forward on the bullet, which will affect its stability no matter the rpm.

I think most people would be better served by the accuracy advantage of "enough" twist rather than purposely over twisting in the hope of the few times the extra spin MAY help a subsonic transition at an ELR target...unless of course this rifle will be a test bed and you will be shooting at targets out where the bullet is subsonic most of the time. In that case, keep good data and let us know what you find out and compare it to the results on those targets from barrels with more typical twists. My guess is the difference will be lost in the noise and that bullet shapes are more important than the rpm of the bullet but the results from a good controlled test would be interesting.

 

[KYpatriot]

^^this^^ and having a 1:8 308 and a few 1:7 223 I can tell you its not the next leap forward in my experience. The 223 does well with bullets that call for it but the lighter weights don’t group nearly as tight. On the 308 I have yet to find anything that will shoot consistently well. One trick that has helped came from Frank Green of Bartlien (spl) barrels and that is you MUST watch your runout when shooting faster than needed twists. Doing that and using the thickest necked brass to more closely align the bullet in the chamber shrunk my groups and got rid of the crazy fliers I was experiencing but I am still not shooting as well as I know I am capable of. What’s the longest bullet you wish to shoot from a 300? If it’s the berger 230 they say (berger) a 1:10 will work, so if it were me I’d go no faster than 1:9.5...

But feel free to experiment.

It may just be that my barrel isn’t perfect or as Todd Hodnet put it in his article that I read, paraphrasing here, “it hasn’t come into its own” yet as his did after 200 rounds.

FWIW, the 308 barrel I have will shoot moa now and occasionally group a few near the .7’s. What gives me pause is if I screw that barrel off and screw on a 1:10 or 1:12 I am half that with occasionals in the .3’s be it a 100 or out long if you factor out my horizontal dispersion from shooter induced screw ups on wind calls. I see this as well with the 223 1:7’s when I go from 69-80 grain bullets down to the low 50-53 grain weights, groups grow to about 100% larger.

This post is a good example of what I was talking about. The "runout" on a loaded round causes the bullet to enter into the barrel slightly crooked, and when that bullet is spun at thousands of rpm the mass imbalance causes wobble and moves the bullet out of the group. Even if you load the ammo with zero runout, the bullets themselves will not be perfectly balanced although modern bullets have come a long way. Obviously if you spin a bullet too fast it can come apart after leaving the barrel which is sometimes seen with varmint bullets.

If Niles is shooting a 155gr bullet at 2900fps out of his 8 twist 308, he is putting 261,000 rpm on the bullet. (2900x(12/8)x60)
With a 11.25 twist it is 185,600. The 8 twist Imparts an extra 75000 rpm, or 1256 revolutions per second over the base rate of the traditional 11 twist, so any manufacturer bullet imperfections or misalignment gets highlighted. If we need the extra twist then we need it and can't help it, but it does have drawbacks.

 

[6.5 GUY]

Niles and KY,

Your statements both have a good deal of validity, and maybe too much of a good thing isn't the answer? I was mainly concerned with the effects when faced with the bullet going subsonic at distance, and maybe increasing the effective range of a given caliber. And whether or not it would help, in the same sense the .223 twist rates kept increasing for all the heavy for caliber bullets that just keep getting even heavier. I thought if still pushed fast enough, velocity wise, the twist would impart enough rotational spin to help overcome those effects at extreme distances?

 

[BryanLitz]

Faster than necessary twist rates have many effects, but most are subtle.

Something like a 1:8" .308 Win shooting 175's can produce very slightly higher BC's thru supersonic (on the order of 1%), and noticeably higher BC's thru transonic (I've measured greater than 5% difference going from 1:12" to 1:8"). The effect isn't so dramatic in ballistic performance, as in you'll see any less wind drift, but where it can matter is with drop predictions being off. But before you consider this a major issue, ask yourself how much shooting you do thru transonic.

Another effect is on precision (groups), based on how well balanced the bullets are. You won't see groups affected by twist so much for well balanced bullets, but for poorly balanced bullets you can see noticable growth in groups with faster twist. Over a range of 1:8" to 1:12", I've measured group averages (again, 308's with 175's) that ran from like .9" to .6" for poorly balanced bullets, but for well balanced bullets the average only ran like .6" to .5" (something like that, I'd have to check my notes to be sure on the exact numbers).

Someone brought up a good point about the effect of run-out in fast twist. That makes sense also as misalignment would cause more dispersion in faster twists. I'll have to run a test on that as well.

Some people worry about losing muzzle velocity with faster twist. Testing shows this just isn't the case, at least not at a level that's significant. I think it was like 1 or 2 fps per inch of twist.

-Bryan

 

[wilwith1l]

^^^ Thank you for actual empirical data.

 

[DAVETOOLEY]

I'll add my 2 cents to what has been said by three smart guys. In 20 years of ELR shooting I have never seen any issues with stability in the transonic zone with bullets that were 1. on the heavier side for a specific caliber and 2. Designed with extended range shooting from the start. Meaning improved BC's.
30 cal. example. Has anyone ever heard of problems with a 190 gr. SMK? I haven't. Not a new design but between weight and design there are no problems at extended ranges.
Fast twist barrels require better bullets to achieve the same level of accuracy. That is based on my definition of accuracy which may be different than yours..

 

[bogeybrown]

And in fact did, including using a 1:7.8 for his .308. He had a prototype .308 rifle that had a 14" barrel and an 1:8 twist. it had no problem hitting a target multiple times at a mile w/the 168 grain ammo. The other rifles had to switch to using the 175 grain ammo to hit that far.
He also feels the 338 lapua should be using a 1:8 twist for the bullets out there.

Todd's 1:7.8 reportedly had 14,000 rounds before it shot out. He also had nearly 4,000 rounds on two 338 LM rifles that were still 1/2" @ 100m and wasn't seeing excessive wear. These could in theory also produce excellent accuracy at extreme ranges at reduced velocities which would extend barrel life.

This was discussed at the Accuracy 1st course a few years ago, and at that point they were happy with their preliminary results at LR with the 1:8 .308 barrels but weren't ready to "commit" to the idea yet until they'd done a lot more testing.
In their fashion of refusing to accept conventional "wisdom" until proving it themselves, they said that when they got good initial results from 1:8 that they had a 1:4 .308 barrel made. That's not a typo, they actually tried a 1:4. They wouldn't go into details, but simply said that "bad things happened" and they stepped back up the 1:8.

 

[6.5 GUY]

Faster than necessary twist rates have many effects, but most are subtle.

Something like a 1:8" .308 Win shooting 175's can produce very slightly higher BC's thru supersonic (on the order of 1%), and noticeably higher BC's thru transonic (I've measured greater than 5% difference going from 1:12" to 1:8"). The effect isn't so dramatic in ballistic performance, as in you'll see any less wind drift, but where it can matter is with drop predictions being off. But before you consider this a major issue, ask yourself how much shooting you do thru transonic.

Another effect is on precision (groups), based on how well balanced the bullets are. You won't see groups affected by twist so much for well balanced bullets, but for poorly balanced bullets you can see noticeable growth in groups with faster twist. Over a range of 1:8" to 1:12", I've measured group averages (again, 308's with 175's) that ran from like .9" to .6" for poorly balanced bullets, but for well balanced bullets the average only ran like .6" to .5" (something like that, I'd have to check my notes to be sure on the exact numbers).

Someone brought up a good point about the effect of run-out in fast twist. That makes sense also as misalignment would cause more dispersion in faster twists. I'll have to run a test on that as well.

Some people worry about losing muzzle velocity with faster twist. Testing shows this just isn't the case, at least not at a level that's significant. I think it was like 1 or 2 fps per inch of twist.

-Bryan

Wow, from one Michigander to another I never thought I'd get an answer to my question from one of the smartest people in the room! Speaking of which, thanks for all the great work you provide us through all your testing and put into words for us to learn more from Brain. You've help propel the art of long range shooting for the vast majority of civilian shooters with your App, better bullets, and your literary works you've written.

Thanks for the response Brian, and Best wishes in the future.

 

[6.5 GUY]

I'll add my 2 cents to what has been said by three smart guys. In 20 years of ELR shooting I have never seen any issues with stability in the transonic zone with bullets that were 1. on the heavier side for a specific caliber and 2. Designed with extended range shooting from the start. Meaning improved BC's.
30 cal. example. Has anyone ever heard of problems with a 190 gr. SMK? I haven't. Not a new design but between weight and design there are no problems at extended ranges.
Fast twist barrels require better bullets to achieve the same level of accuracy. That is based on my definition of accuracy which may be different than yours..

Dave,

You wouldn't remember me but over a decade ago I had spoke to you and your Co-worker in length about building me a hunting rifle in a 7mm Boo Boo. I did acquire one used after that, but after many years in the field converted it to a 280 AI for the ease of finding brass. That gun was an absolute lazer beam when shot! I loved it and regretted changing it over.

This project is just in the thought process, and is not cast in stone in my mind. I'm just looking for some answers so that I don't regret the rebuild in the end.

Hope you are doing well and best wishes to you Dave.

 

[DAVETOOLEY]

Dave,

You wouldn't remember me but over a decade ago I had spoke to you and your Co-worker in length about building me a hunting rifle in a 7mm Boo Boo. I did acquire one used after that, but after many years in the field converted it to a 280 AI for the ease of finding brass. That gun was an absolute lazer beam when shot! I loved it and regretted changing it over.

This project is just in the thought process, and is not cast in stone in my mind. I'm just looking for some answers so that I don't regret the rebuild in the end.

Hope you are doing well and best wishes to you Dave.

I'm doing well. I always liked that round. I have a switch barrel rifle in 7MM Tooley and 338 Tooley, same case. Now we have the 375 Ruger case to work with.