Effects of gain twist barrels on chamber pressure and velocity

Nathan1990

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Dec 2, 2021
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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

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    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

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    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

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    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

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    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.
     

    badassgunworks

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    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.
     
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    Nathan1990

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    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

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    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
     
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    badassgunworks

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    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
     
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    sirhrmechanic

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    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

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    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
     
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    Nathan1990

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    Dec 2, 2021
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    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
     
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    sirhrmechanic

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    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

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    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.
     
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    badassgunworks

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    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.
     
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    TripleBull

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    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

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    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.
    Barrel%20rifling
     
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    sirhrmechanic

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    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.
    Barrel%20rifling
    ^^^ 5" Naval gun barrel?

    Sirhr
     

    oneshot.onehit

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    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

    78B27789-BF31-448E-BDBB-1E2442F5B1CA.png
     

    jasent

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    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

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    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.
     
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    Baron23

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    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.