Gain Twist can it reduce pressure?

[jimfinn]

I was told that having a fast twist,say 7.5' in a 6.5mm, will result in higher pressure than say a 1:9' twist given the same load and bullets. If that is true, would an advantage of a gain twist say starting with 1:9' and gradually increasing until you have the last 4'-6' of the bore at 1:7.5', give you less pressure than a straight 1:7.5' twist? And if this is true, then wouldn't this barrel last somewhat longer than a straight fast twist one?
Is this a possible advantage of going to a gain twist?
I am ordering a 6.5mm barrel that will be chambered as a 260 or 260AI. I'd like to have the option of sometimes shooting long bullets, such as the 140 A-Max, that require a fast twist.
I must confess that I like trying things I never had before, and although gain twist has been around for over a hundred years, I have never owned a barrel with it.
Thanks
Jim

 

[tightneck]

I was told that having a fast twist,say 7.5' in a 6.5mm, will result in higher pressure than say a 1:9' twist given the same load and bullets. If that is true, would an advantage of a gain twist say starting with 1:9' and gradually increasing until you have the last 4'-6' of the bore at 1:7.5', give you less pressure than a straight 1:7.5' twist? And if this is true, then wouldn't this barrel last somewhat longer than a straight fast twist one?
Is this a possible advantage of going to a gain twist?
I am ordering a 6.5mm barrel that will be chambered as a 260 or 260AI. I'd like to have the option of sometimes shooting long bullets, such as the 140 A-Max, that require a fast twist.
I must confess that I like trying things I never had before, and although gain twist has been around for over a hundred years, I have never own a barrel with it.
Thanks
Jim

First, the premise that a faster twist will increase pressure is unfounded. While gain twists have been around for a long time, its only recently that they are really being put to the test in small arms. I think we need a LOT more data before any real conclusions are drawn. I think its pretty apparent that they aren't hurting anything, but are they the next great thing? Only time will tell...

 

[jimfinn]

Thanks,
This why I asked here. Like I wrote, the higher pressure notion is just what a fellow shooter told me. I don't know where he got it from. If the advantages of gain twist for modern cartridges are not known, I can save some money by just going to a uniform twist.
Jim

 

[rcw3]

First, the premise that a faster twist will increase pressure is unfounded....

I don't know what data you have to conclude that, but I would absolutely disagree with that. A tighter twist definitely increases pressure and I have found you can employ a gain twist to help accomplish things that it is hard to with a straight twist,particularly when you need a very tight twist). This is particularly so when you are working with bullets that are big for the bore size and need a very fast twist for stability,i.e. 6mm 115's, 224 90 gr bullets, etc.).

There's always a 'point of no return' with barrel twists where the twist is so tight it becomes like a wall,rather than a ramp) when a bullet has to go from zero to 250000+ rpm in fractions of a second. The closer you get to the edge, the more pressures rise sabotaging velocity. For a 6mm, a 7.5' twist is very close to the edge and same with a 6.5' Twist for a .224 bore. With a gain twist you can start off at the breech with a more relaxed twist to keep your pressures down and the velocity up and have the bullet rpms accelerate as the bullet travels down the bore rather than having to do it all at once as the bullet engraves on the lands and has to go up to full rpms almost instantly with a straight twist barrel.

I am not saying these gain twists are the be all and end all, but there is a useful place for them for sure.

Robert Whitley

 

[tightneck]

First, the premise that a faster twist will increase pressure is unfounded....

I don't know what data you have to conclude that, but I would absolutely disagree with that. A tighter twist definitely increases pressure and I have found you can employ a gain twist to help accomplish things that it is hard to with a straight twist,particularly when you need a very tight twist). This is particularly so when you are working with bullets that are big for the bore size and need a very fast twist for stability,i.e. 6mm 115's, 224 90 gr bullets, etc.).

There's always a 'point of no return' with barrel twists where the twist is so tight it becomes like a wall,rather than a ramp) when a bullet has to go from zero to 250000+ rpm in fractions of a second. The closer you get to the edge, the more pressures rise sabotaging velocity. For a 6mm, a 7.5' twist is very close to the edge and same with a 6.5' Twist for a .224 bore. With a gain twist you can start off at the breech with a more relaxed twist to keep your pressures down and the velocity up and have the bullet rpms accelerate as the bullet travels down the bore rather than having to do it all at once as the bullet engraves on the lands and has to go up to full rpms almost instantly with a straight twist barrel.

I am not saying these gain twists are the be all and end all, but there is a useful place for them for sure.

Robert Whitley

I'll tell you where I got it. My own experience as well as talking to Frank Green. Frank indicates that of all the customers he talks to about the fast vs slow twist, gain vs non gain, most see zero difference in velocities. I don't have a bunch of experience with gains, but I do have a very radical gain twist that shows real promise. SAME velocity range as several Bartliens with standard twist rifling, same land and groove dimensions. Matter of fact, the gain twist ends up at a slower twist than any of the standard twist barrels. I have another not so radical gain that shows the same results so far.

 

[HPMike800]

Jim:
If your looking to shoot 6.5 140's out of a 260 rem, 1:8.5 works fine.

I would really like to know the results from you gain twist guy's on long 115gr bullets. My thought on gain twist is that the continual re-engraving of the bullet would add pressure as you go down the pipe.,esp with long bullets). I guess my hope is that the pressure curve would be flatter, resulting in better brass life and more reliable performance in long strings of fire.

Mike

 

[rcw3]

tightneck

Funny that my experience differs from yours and it appears we are both using Bartlein barrels,and I just ordered 4 more gain twist barrels from Bartlein a couple weeks ago because I felt there was merit in pressure reductions with a gain twist). I would note that you made a blanket statement without any specifics or qualifications. I cited specific circumstances where I see the gain twist as having a good application.

I am not going to cite my dicussions with Frank Green, but suffice it to say, based on my discussions with Frank, I have some different recollections, so maybe it's best we both leave the hearsay,and Frank) out of it and just stick to facts and data.

Robert Whitley

 

[rcw3]

Mike

It seems most people tend to think of a bullet going down the bore like the hunk of hard metal they hold in their hand when they re-load. When it's subjected to the pressures and forces of being shot through the barrel I believe it's a lot more plastic than many realize or conceptualize and based on what I see from my testing, I believe a bullet will conform a lot easier to a gain twist as it travels down the bore than one might think.

Robert Whitley

 

[GerryM]

I HAVE A GAIN ON ORDER FROM FRANK.
IT WILL BE CHAMBERED IN 6PPC FOR TESTING.
I MAKE MY OWN BULLETS AND THIS IS JUST AN EXPERIMENT TO SEE WHAT CAN BE GAINED WITH THE GAIN TWIST.
RIGHT NOW MY 8 OG BULLETS ARE SHOOTING IN THE LOW ONES IN A FIVE SHOT GROUP. I'M HOPING THIS MAY CHANGE TO EVEN SMALLER GROUPS WITH THE GAIN TWIST. SOME WHERE IN THE PRESSURE VELOCITY CURVES LIES THE ANSWER.
I KNOW FRANK WILL DELIVER A HIGH QUALITY BARREL . THAT IS ONE VARIABLE THAT IS ELIMINATED.
IF IMPROVEMENT'S HAPPEN I'LL POST RESULTS.
THE TESTS WILL BE WITH BOTH BULLETS THE 68 WITH THE 7 1/2 OG AND THE 8 OG

 

[GerryM]

I see where your going on your questions'
The proof will be in the group sizes.
We know about the two curves and accuracy.
That said the laymen here are not equated with the hypothetical points . Their proof is in the shooting.
The gain I have ordered is a 14.25 going to 13.25 twist.
just about 1 twist more on exit. Right now the 13 1/2 is doing fine.
I believe a tad more spin won't hurt and the point of starting out slower may help in the final tests.
The bearing surfaces are similar but different in stability on the two dies. The 8 is nearly perfect. Now if i can improve with the 7 1/2 that will be really something also. That particular bullet shape also shoots very small group,s . I have seen some .085 and less at 100 yids.
I have to see what effect the gain has on the actual shape of the bullet's {lands marks]. .
The questions are many and at least one barrel maker doesn't think it has merit. He may be right, But i like to find out things for my self.
One example is the difference between button rifling stress and cut rifling. The conclusions on that are out too.

 

[Jeffvn]

I see that lots of us have been bending Frank's ear about the gain twist barrels that they are no putting out. I should have in hand very shortly a gain twist 338 barrel for testing some ultra high BC Banded solid bullets.

In my discussions with not only frank but also the bullet designer, several thing come out. These are my words, and I'm nobody's expert on this but here goes. We hope,I said HOPE - because not all of this has proven itself to be true as yet) that the gain twist will reduce some of the pressure by starting at a twist slower than needed to stabilize the bullet but tightening down as the bullet progresses down the bore,in my case a 12 twist and exiting the muzzle at a 5.4' - not a typo). Additionally, the gain twist will interface with the drive bands of the bullets at slightly different points as it progresses down the bore, allowing for a slightly different band design to be used.

The theory of reduced pressure makes some sense to me, as I can see how a bullet progressing down the bore of a 12 twist results in different resistance/pressure to a given load than one going through a 5.4' twist,the wall that Robert talked about). All in all, I think this could be a dramatic step forward, but we'll have to see. I promise to post my result to keep everyone apprised of my progress anyway.

JeffVN

 

[jimfinn]

Hey All,
Thanks for all the good points of view and information. I chewed on it for awhile and asked a few other folks. Then after I got with Frank over this I decided on a straight 1:8 twist 5R 6.5mm barrel.
Frank said that what he thinks is the thing folks are starting to see with gain twists is a wider tune range with there loads. As this rifle will be used for hunting, he convinced me it most likely wouldn't be enough of an advantage for me to justify the extra cost. Now that is someone looking out for their customer!
Hopefully this will turn into a Fall/Winter project.
My Best to All!
Jim

 

[cliffy]

A 1-in-12' twist .223 Remington twixt 36 and 60 grain fare works to perfection. Hampering these varmint loads with FAST twisters aids nothing, and perhaps inhibits downrange performance. If'n I opted for 75 grain Swift Scirocco IIs, I'd look seriously at 1-in-8' barrels. While this particular bullet could oft a whitetail, I doubt it would be a proper coyote-choice bullet, unless one wanted a hole blown through both sides and lots of space needed to stop the bullet beyond the hit. I find the Barnes Varmint Grenade 50 grain difficult enough to stop, but it does tend to serve well on any Coyote stalked and served fair notice. Valuable pelts require proper bullets. A 60 grain Nosler Partition bullet will kill nearly anything, but a pelt may end-up being worth $1.20 opposed to $30 plus dollars using a sane bullet choice. Cliffy