Effects of gain twist barrels on chamber pressure and velocity

[Nathan1990]

So i recently learned about gain twist barrels and wondering has anyone out there done extensive testing on them in correlation to chamber pressure and velocity cause i know a more aggressive twist barrel increases friction/forces on the bullet therefore increasing chamber pressure and conversely lower twistrates have less friction/forces reducing chamber pressure
So lets say in a prefect world where you had the ability to make a barrel in whatever twistrate you wanted and you had a physics engine that could simulate the chamber pressure when the bullet is at any point in the barrel during the firing process
So someone could theoretically tune a gain twist barrel to make to to where you could fire heavier bullet with an above max load of powder but start with a very slow twist rate like a 1-12 to keep the chamber pressures to an acceptable level but increase the twist rate quickly to let say 1-6 for the sake of discussion to increase chamber pressure as it goes down the barrel to try and keep the chamber pressure curve more flat (as the gasses expand) and keep the force pushing the bullet down the barrel more consistent and have more total force acting on it through out the length of the barrel without increasing chamber pressure above acceptable levels therefore allowing you to use more powder cause the slow twist rate at the beginning keeping pressures down to an acceptable level and then try and keep the chamber pressures higher further down the barrel using a gaining twist rate and thus increasing velocity without increasing chamber pressure i think this would also help with more complete powder burn as well cause your trying to control that pressure wave of expanding gasses using the forces on the bullet from your gaining twistrate versus on a normal barrel where ur chamber pressure is at it’s highest right when the primer is ignited basically goes down progressively as the bullet moves down the barrel and the gasses expand

Idk im not gunsmith and i’m no physics expert but learning about gain twistrate barrels had me thinking on the theoretical possibilities and what you could do with custom gain twistrate barrels

 

[OREGUN]

Wow.

 

[Nathan1990]

Wow.

Ik i mightve started think a little too deeply about it

 

[OREGUN]

Ik i mightve started think a little too deeply about it

A little punctuation would help too. I started to feel like I was the one doing cocaine when I got to the end of the first “sentence”.

 

[OREGUN]

But seriously, I don’t know enough about the theoreticals here to give you an answer. I shoot a gain twist but right now I’m feeding it factory ammo…and I’ve never reloaded the caliber to have any idea about the pressures encountered or max loads available.

 

[Nathan1990]

A little punctuation would help too. I started to feel like I was the one doing cocaine when I got to the end of the first “sentence”.

But seriously, I don’t know enough about the theoreticals here to give you an answer. I shoot a gain twist but right now I’m feeding it factory ammo…and I’ve never reloaded the caliber to have any idea about the pressures encountered or max loads

 

[Nathan1990]

A

 

[Nathan1990]

Idk i wish there was a ballistics calculator out there advanced enough to tell you exact pressures in the chamber, at any given point in the firing process, with any given powder/amount grain bullet and twist rates ect.

 

[6.5SH]

Idk i wish there was a ballistics calculator out there advanced enough to tell you exact pressures in the chamber, at any given point in the firing process, with any given powder/amount grain bullet and twist rates ect.

Exact? No.

An often decent simulation? QuickLoad and Gordon's Reloading Tool.

 

[Nathan1990]

Exact? No.

An often decent simulation? QuickLoad and Gordon's Reloading Tool.

Ill have to give that a look

 

[badassgunworks]

Think of of this way. a barreled action Works like a air compressor. A compressor tank like your cartridge.The hose or line like your barrel. You may have 175psi on your 80 gallon tank but you will never maintain 175 psi in a line when running a power tool. Pressure in your chamber may be 60,000 psi but 10" down your barrel behind your moving bullet pressure is only about 15,000 psi. It works not like you think. . Increasing twist rate wont change much on your velocity. Sustained force pressure is never dictated by barrel. only cfm of cartridge volume and presssure limits , burn rate and bore diamiter, burn characteristics of powder.

 

[Nathan1990]

Think of of this way. a barreled action Works like a air compressor. A compressor tank like your cartridge.The hose or line like your barrel. You may have 175psi on your 80 gallon tank but you will never maintain 175 psi in a line when running a power tool. Pressure in your chamber may be 60,000 psi but 10" down your barrel behind your moving bullet pressure is only about 15,000 psi. It works not like you think. . Increasing twist rate wont change much on your velocity. Sustained force pressure is never dictated by barrel. only cfm of cartridge volume and presssure limits , burn rate and bore diamiter, burn characteristics of powder.

I guess Im more saying can you try to use your twistrate as a tuning device to try and control that pressure wave a little bit to make that pressure curve flatter and make higher pressures for longer down the barrel while keeping it safe at the beginning
And use above a max load of powder thanks to the slow twist rate at the beginning lowering the pressure marginally at the beginning

 

[badassgunworks]

I guess Im more saying can you try to use your twistrate as a tuning device to try and control that pressure wave a little bit to make that pressure curve flatter and make higher pressures for longer down the barrel while keeping it safe at the beginning
And use above a max load of powder thanks to the slow twist rate at the beginning lowering the pressure marginally at the beginning

No.... gain twist barrels only reduce bullet stress. Highest pressure spike is when bullet engages lands and grooves during the conforming prosess. That spike is on avrage 10,000 psi. a bore rider throat is the best way to obtain what you are wanting to see. Not gain twist. More powder with out the ability to engage easier only increases the engagement spike . increased ramping of velocity is better obtained threw longer barrels threw duration

 

[badassgunworks]

Primary advantage of gain twist is reduced bullet damage so that you can higher volumes of powder at higher pressures so you can run longer barrels to obtain higher
velocities with heavier higher bc bullets. But the gain twist does not increase velocity by design in fact it may reduce velocity

 

[sirhrmechanic]

Gain twist barrels have been around since the days of the patched round ball… because they work really well for their intended purpose.

This is to impart more spin on the projectile by gradually increasing its rate of spin over time/distance.

A projectile that would “strip” on hitting 1:7 twist rifling can be accelerated to 1:7 by starting at 1:10 and “gradually” increasing the spin rate over the length of the barrel. That all this take place in a fraction of a second is irrelevant. It is still gradual from an internal ballistics standpoint.

The gain twist also lets the bulket enter the rifling at a shallower angle, keeping it more in alignment with the center axis of the bore from the moment of engagement

Gain twist barrels won’t cause spikes or pressure issues. They will let you stabilize longer and heavier projectiles, however. And this offers better accuracy and repeatability.

One of the reasons gain twist barrels are coming back in vogue (really they never went away… they were just a niche thing) is because they offer fantastic advances in long range precision shooting. The disadvantage has always been cost, but new manufacturing methods make the costs bearable now. And top barrel makers like Bartelin are offering superb gain twist barrels. I am sure @Frank Green or others can comment in much greater detail as the real experts are here on this forum.

My two most recent rifle orders are for gain twist barrels, BTW. I’ve been waiting ages for the industry to catch up with history and in the last few years, huge gains have been made.

Gain twist is a fantastic technology. But there is nothing newfangled or scary about it. Colt Army and Navy revolvers used it. GAU Gatling gun barrels still use it. Daniel Boone could probably have told you how it works. He might have “Kilt a Bar” with one!

Cheers, Sirhr

 

[Nathan1990]

Primary advantage of gain twist is reduced bullet damage so that you can higher volumes of powder at higher pressures so you can run longer barrels to obtain higher
velocities with heavier higher bc bullets. But the gain twist does not increase velocity by design in fact it may reduce velocity

That makes allot of sense i was just way overthinking it

 

[Nathan1990]

Gain twist barrels have been around since the days of the patched round ball… because they work really well for their intended purpose.

This is to impart more spin on the projectile by gradually increasing its rate of spin over time/distance.

A projectile that would “strip” on hitting 1:7 twist rifling can be accelerated to 1:7 by starting at 1:10 and “gradually” increasing the spin rate over the length of the barrel. That all this take place in a fraction of a second is irrelevant. It is still gradual from an internal ballistics standpoint.

The gain twist also lets the bulket enter the rifling at a shallower angle, keeping it more in alignment with the center axis of the bore from the moment of engagement

Gain twist barrels won’t cause spikes or pressure issues. They will let you stabilize longer and heavier projectiles, however. And this offers better accuracy and repeatability.

One of the reasons gain twist barrels are coming back in vogue (really they never went away… they were just a niche thing) is because they offer fantastic advances in long range precision shooting. The disadvantage has always been cost, but new manufacturing methods make the costs bearable now. And top barrel makers like Bartelin are offering superb gain twist barrels. I am sure @Frank Green or others can comment in much greater detail as the real experts are here on this forum.

My two most recent rifle orders are for gain twist barrels, BTW. I’ve been waiting ages for the industry to catch up with history and in the last few years, huge gains have been made.

Gain twist is a fantastic technology. But there is nothing newfangled or scary about it. Colt Army and Navy revolvers used it. GAU Gatling gun barrels still use it. Daniel Boone could probably have told you how it works. He might have “Kilt a Bar” with one!

Cheers, Sirhr

Thank you was just trying to learn more about them before i spend the money and try them for myself ill have to call the guys over at bartelin on what twist i should go with for my rifle

 

[sirhrmechanic]

Thank you was just trying to learn more about them before i spend the money and try them for myself ill have to call the guys over at bartelin on what twist i should go with for my rifle

Oh and welcome to SH... As first posts/first day's questions go... you have a good one! Don't worry about the grammar Nath-zees too much. Though you will find we are a community dedicated to 'precision' in many forms. Including quality of communications!

I am following this post because it's sometimes the simple questions where you learn the most.

Cheers and, again, welcome aboard.

Sirhr

 

[Ledzep]

The amount of "over max" you'll be able to go by having a gain twist barrel because of the difference in initial spin rate vs. final spin rate next to one that just starts and ends at the "final" spin rate is going to be negligibly small. I'd guess within the window of calibration error on a P&V barrel.

Doing drastic changes, like 1:12 down to 1:8 or some such will likely cause problems with bullets that don't use driving bands.

Gain twist is an interesting thing to think about, but in my opinion the application can have most advantage taken out of shooting large caliber monolithic bullets with driving bands.

I should sit down some time and try to estimate and plot the rotational acceleration curves for traditional and gain twists. I question whether or not it makes a meaningful difference in shoulder-fired sized cartridges... The rate of growth of volume vs. diameter makes sense for artillery having gain twist.. a 16" 2700lb projectile is 60.6 X the diameter but 128,571 X the weight of a 6.5mm 147 ELD... so rotational acceleration vs. linear acceleration and the inertias involved as compared to 'static' material properties start to matter at some point in between... In small caliber stuff I'm not sure it makes a meaningful difference at all. I've shot out a couple of gain twist barrels and a lot of traditional, and from the end user standpoint I can't say there was really anything that stood out from the norm.

 

[badassgunworks]

The amount of "over max" you'll be able to go by having a gain twist barrel because of the difference in initial spin rate vs. final spin rate next to one that just starts and ends at the "final" spin rate is going to be negligibly small. I'd guess within the window of calibration error on a P&V barrel.

Doing drastic changes, like 1:12 down to 1:8 or some such will likely cause problems with bullets that don't use driving bands.

Gain twist is an interesting thing to think about, but in my opinion the application can have most advantage taken out of shooting large caliber monolithic bullets with driving bands.

I should sit down some time and try to estimate and plot the rotational acceleration curves for traditional and gain twists. I question whether or not it makes a meaningful difference in shoulder-fired sized cartridges... The rate of growth of volume vs. diameter makes sense for artillery having gain twist.. a 16" 2700lb projectile is 60.6 X the diameter but 128,571 X the weight of a 6.5mm 147 ELD... so rotational acceleration vs. linear acceleration and the inertias involved as compared to 'static' material properties start to matter at some point in between... In small caliber stuff I'm not sure it makes a meaningful difference at all. I've shot out a couple of gain twist barrels and a lot of traditional, and from the end user standpoint I can't say there was really anything that stood out from the norm.

There is a possibility that the slower start on the Progressive twist increase will reduce the possibility of jacket damage. Witch may reduce BC SD. However on your aggressive gain twist barrels it can also cause a wider Groove imprint. That could also slightly reduce the ballistic coefficient of your bullet. What it comes down to. There are pros and cons to all design changes. It just depends on desired end results.

 

[TripleBull]

The amount of "over max" you'll be able to go by having a gain twist barrel because of the difference in initial spin rate vs. final spin rate next to one that just starts and ends at the "final" spin rate is going to be negligibly small. I'd guess within the window of calibration error on a P&V barrel.

Doing drastic changes, like 1:12 down to 1:8 or some such will likely cause problems with bullets that don't use driving bands.

(snip)

I can't remember if I read a post by Frank or if it was an instructional video about GT, but when I ordered a barrel from Mile High, they advised I go with a max twist difference of about 1. I followed that advice and Bartlein delivered it (eventually...). It's a shooter and I'm happy as a pig in shit.

 

[Ledzep]

I think I did a 1:9.5 to 1:8.5 .300 PRC barrel. Shot well, but not demonstrably any better than my 1:8.5 that's on it now. Same story for a couple of 6.5 barrels. I'm not advocating against it, I just don't know that it matters for the small caliber stuff.

I'm not especially well versed in large caliber internal ballistics but my understanding is that at a certain point it becomes necessary, in the larger cases it get's pretty dramatic.

 

[sirhrmechanic]

I think I did a 1:9.5 to 1:8.5 .300 PRC barrel. Shot well, but not demonstrably any better than my 1:8.5 that's on it now. Same story for a couple of 6.5 barrels. I'm not advocating against it, I just don't know that it matters for the small caliber stuff.

I'm not especially well versed in large caliber internal ballistics but my understanding is that at a certain point it becomes necessary, in the larger cases it get's pretty dramatic.

^^^ 5" Naval gun barrel?

Sirhr

 

[oneshot.onehit]

From a few years back, this barrel is a laser for the individual shooting it now with a different 361 solid today. Pictured is a 414 grain recovered - 16 to 7.7 gain twist - all about maxing a load and not pressure, for all those that claim it tears a bullet up. The bullet looks dang good to me.

Cheers
oneshot

 

[jasent]

From a few years back, this barrel is a laser for the individual shooting it now with a different 361 solid today. Pictured is a 414 grain recovered - 16 to 7.7 gain twist - all about maxing a load and not pressure, for all those that claim it tears a bullet up. The bullet looks dang good to me.

Cheers
oneshot

View attachment 7755324

Definitely looks good to me

 

[mojavejim]

I have a few Bartlein gain twists, couple 308's didn't notice any reduced pressures from std. Bartlein barrels. They do seem to be easier and more forgiving for load development. All my gain twist are only .75 gain as recommended by Bartlein. An interesting note though is with the 416 pushing 500gn. A-tips around 3000fps w/gain my bullets stay together & my buddy's with a std. twist at the same velocities seem to "evaporate" randomly.

 

[Baron23]

I just sent a Bartlein 6.5 cm MODBB left gain twist barrel to Altus to be spun and mounted on a Deviant.

I too went w .75….8.25-7.5.

I’m not expecting miracles from it but hopefully it will be a bit more gentle w my bullets which will be somewhat helpful.

 

[Anthony Laversa]

Interesting. Simliar topic I started, we may be able to help each other See: https://www.snipershide.com/shootin...rfect-barrel-and-internal-ballistics.7120172/

 

[Kinetic Moose]

OK,
so i'm reading and I have a brain that wanders in the night, bumps into things and then it's morning and I haven't gotten any sleep, so let me inject this food for thought from my physics background.

So let's talk basic physics, free body diagram type.
I have a bullet that is at rest, and i want to start spinning it, as fast as possible in a meter while coincidentally increasing the velocity of the projectile. Logic says overcome static friction first, then while the bullet is moving slowly begin the rotation as the friction coefficient of the barrel lands will be less at a lower energy level, so spin it fast early in the barrel, then as it velocity through the barrel increases you can decrease the twist reducing the friction input into the angular momentum thereby allowing more speed gain and possibly a calmer barrel exit as reversing the twist increase factor(gain barrel twist we all think of) uses the velocity of the projectile through the barrel to increase the angular momentum at the very end of the barrel, thereby putting a lot more energy into the end(read unsupported) end of the barrel increasing harmonics, stretch, elongation etc.

The CATCH, in an ideal world, we would spin the projectile up to stabilize it before starting its horizontal journey, giving it the most stability before increasing the external forces acting on it through traveling the length of the barrel, problem is powder, we would need a powder that would burn very slowly to keep chamber pressures low enough at first and then steadily climb to get maximum velocity out of it at the end, and from what i understand, powders don't work that way. So therein lies the balance, enough spin to stabilize it while maximizing speed and without destroying things to boot.

So what's the answer, well logic also says that necked down cartridges wouldn't be a good choice because they build pressure FAST. Then as you approach a 1:1 ratio of cartridge diameter to bore, that problem is reversed, only to be replaced by volume issues, can you fit enough slow burn into a cartridge to spin this thing up and still push it at max V.

I've wrestled with this enough to ponder what gain twist I'd like to get, maybe a 37 caliber, with a starting of 8 reducing to a 9.5, which leads me back to physics, if you spin it fast in the beginning, you have to make sure that you don't LOSE RPM along the length of the barrel as this will be reduced DIRECTLY back into angular momentum reduction causing friction and turning the whole thing into a waste of metal.

My guess is someone has tried both ways, but searching I can't find any mathematics on it and I haven't had the time to devote to trying to figure out the acceleration curves of lateral v angular yet.

Sorry if this post offends anyone, I'm just trying to learn about this and may build an elr rig sometime.

 

[Pete Cefalu]

OK,
so i'm reading and I have a brain that wanders in the night, bumps into things and then it's morning and I haven't gotten any sleep, so let me inject this food for thought from my physics background.

So let's talk basic physics, free body diagram type.
I have a bullet that is at rest, and i want to start spinning it, as fast as possible in a meter while coincidentally increasing the velocity of the projectile. Logic says overcome static friction first, then while the bullet is moving slowly begin the rotation as the friction coefficient of the barrel lands will be less at a lower energy level, so spin it fast early in the barrel, then as it velocity through the barrel increases you can decrease the twist reducing the friction input into the angular momentum thereby allowing more speed gain and possibly a calmer barrel exit as reversing the twist increase factor(gain barrel twist we all think of) uses the velocity of the projectile through the barrel to increase the angular momentum at the very end of the barrel, thereby putting a lot more energy into the end(read unsupported) end of the barrel increasing harmonics, stretch, elongation etc.

The CATCH, in an ideal world, we would spin the projectile up to stabilize it before starting its horizontal journey, giving it the most stability before increasing the external forces acting on it through traveling the length of the barrel, problem is powder, we would need a powder that would burn very slowly to keep chamber pressures low enough at first and then steadily climb to get maximum velocity out of it at the end, and from what i understand, powders don't work that way. So therein lies the balance, enough spin to stabilize it while maximizing speed and without destroying things to boot.

So what's the answer, well logic also says that necked down cartridges wouldn't be a good choice because they build pressure FAST. Then as you approach a 1:1 ratio of cartridge diameter to bore, that problem is reversed, only to be replaced by volume issues, can you fit enough slow burn into a cartridge to spin this thing up and still push it at max V.

I've wrestled with this enough to ponder what gain twist I'd like to get, maybe a 37 caliber, with a starting of 8 reducing to a 9.5, which leads me back to physics, if you spin it fast in the beginning, you have to make sure that you don't LOSE RPM along the length of the barrel as this will be reduced DIRECTLY back into angular momentum reduction causing friction and turning the whole thing into a waste of metal.

My guess is someone has tried both ways, but searching I can't find any mathematics on it and I haven't had the time to devote to trying to figure out the acceleration curves of lateral v angular yet.

Sorry if this post offends anyone, I'm just trying to learn about this and may build an elr rig sometime.

Very interesting. so did you decide to build?

 

[Kinetic Moose]

Very interesting. so did you decide to build?

Currently no, been busy with life at the moment and where I am in the world, no place to send it... within a 2 hour ride. kinda annoying.

 

[Dead Eye Dick]

I discussed this very subject with Frank at Bartlien several years ago. I had an XP-100 Fireball with 1-7 twist. I was shooting 80 grain Bergers and the load went super critical, pressure wise, real quick. (Something like 0.3 grain of IMR4227 went from a load that was too light to have the case conform to the chamber to having serious pressure signs.

My thought was that having a gain twist would take some of the initial pressure and make it easier on the handgun. Frank explained that a gain twist would not accomplish what I was after.

Using very careful loading techniques, the load proved effective against 65 pound steel rams at 200 meters. (They have to be knocked off of the stand, or the hit does not count) However, the little handgun was much happier with 69 grain Sierra’s usign it as a half scale gun for which it was intended.