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Gain twist barrels?
- Thread starter Mattm8725
- Start date
Idea is to stop deformation of jacket.......start with a slower twist, let projectile get started than gradually accelerate twist and torque forces.
Theory is it allows faster exit rotation without excessive torsional force at start.
That seems like a lot of expectation in a milliseconds worth of travel.
I've thought about this question many times. I don't believe it does because the grooves are the grooves and once the bullet has started in the rifling the only difference is that the rotation is gradually accelerated down the bore vs a standard twist rate where the rotation is constant.
The deformation usually occurs on projectiles of lower quality as they engage the rifling on an aggressive twist where the lead core and jacket want to rotate opposite of one another.
Hi,
You can not tell the difference of the engravings on the projectile from super hard GT to normal straight twist.
I will try and find picture here on sh from another elr member that recovered projectile from something like an 11.5 to a 6 twist.
@oneshot.onehit Do you have that picture I am visualizing?
Sincerely,
Theis
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1300 asl. I dont normally load the 208 eld as my typical choice is the 185 juggernaut. I did take the 208's out to 800 with no apparent stability issues.
I’d really like to see the fired projectiles compared between standard twist and gain twist. I’d like to see the wear compared on the individual projectiles.
So I just read this thread from start to finish. I don't have a lot of shooting/reloading experience, but it's something I find super interesting and the researcher in me can't help himself. I am currently saving up some cash to build myself a rifle and get into the reloading game. I work at a lab designing experiments involving fast moving material, normally driven by explosives. I have a couple of thoughts/questions.
Generic questions about what is happening that I think would be interesting to have answers to:
First thing, has there been any experimental work to determine the actual rotation behavior of a bullet as it engages the lands? The two extremes would be just "sliding over" the lands without any rotation initially vs rotating perfectly with the lands as soon is contact is made. How does the ratio of "sliding over" versus rotating with the lands on initial contact change with initial starting twist? (assuming constant load and bullet specs)
Second thing, does the velocity of the bullet, when it engages the lands make a difference? Different combinations of cartridge, power burn rate, bullet weight etc. are bound to change the initial velocity profile of the bullet as it is running into the lands. Can a given bullet withstand a more aggressive initial twist if the velocity that it engages the lands is lower?
Third thing, I saw the comments about checking pressure at different places on the barrel for straight ang gain twist barrels, has anyone tried measuring the velocity of the bullet from start to position to muzzle exit for the different twist arrangements? This could be done using PDV. It would give you the velocity profile of the bullet from the time it was stationary all the way down the barrel. This would make for a great comparison between barrel types. For the test you would need to use a flat faced bullet to be able to get the laser light for PDV to reflect and send the signal back to the probe. But, as far as I can think having a flat face would only really make a difference to the performance to a bullet after in exits the barrel.
Fourth thing, there are places that have very powerful flash x-ray imagining capabilities where it would be possible to actually take x-rays of the chamber/rifling startup area of the barrel at several intervals in time as pressure is building and the round is starting to move. This make be a way to interrogate exactly what the bullet is doing as it engages the lands and begins its travel down the barrel in terms of rotation.
Specific Application question:
I want to build a rifle in 300 PRC. I want to mainly use it for long range hunting (spending a decent amount of time practicing on targets). I plan to use monolithic bullets such as Hammer, Barnes, Cutting Edge etc. My question is, can I use gain twist to help me stabilize some of the really long mono bullets but still be able to shoot the lighter ones? Hammer 152 grain needs 1-12, but 227 grain needs 1-7.5 twist. If they are all mono bullets from the same material is "over spinning" really even an issue? As in, if the 227 grain bullet can take the spin, shouldn't the 152 grain bullet from the same material be able to take the spin?
Second application question, is there diminishing returns here chasing the longer higher BC bullets because of the twist requirements? If you have to achieve the 1-7.5 twist for the higher BC bullet, and assuming you do it with gain twist, is there enough of a chance that you could have a bullet leave the barrel that is damaged from the aggressive twist that it's not worth it? The Hammer 199 grain only needs 1-9.25 twist, maybe doing a gain twist that finished at 8.75 and just ignoring those heaviest bullets is the best option?
I apologize that this post got so long!
And, I apologize in advance for my typos! (I'm ok at doing science, but I am pretty shit at writing it up and proofreading)
Generic questions about what is happening that I think would be interesting to have answers to:
First thing, has there been any experimental work to determine the actual rotation behavior of a bullet as it engages the lands? The two extremes would be just "sliding over" the lands without any rotation initially vs rotating perfectly with the lands as soon is contact is made. How does the ratio of "sliding over" versus rotating with the lands on initial contact change with initial starting twist? (assuming constant load and bullet specs)
Second thing, does the velocity of the bullet, when it engages the lands make a difference? Different combinations of cartridge, power burn rate, bullet weight etc. are bound to change the initial velocity profile of the bullet as it is running into the lands. Can a given bullet withstand a more aggressive initial twist if the velocity that it engages the lands is lower?
Third thing, I saw the comments about checking pressure at different places on the barrel for straight ang gain twist barrels, has anyone tried measuring the velocity of the bullet from start to position to muzzle exit for the different twist arrangements? This could be done using PDV. It would give you the velocity profile of the bullet from the time it was stationary all the way down the barrel. This would make for a great comparison between barrel types. For the test you would need to use a flat faced bullet to be able to get the laser light for PDV to reflect and send the signal back to the probe. But, as far as I can think having a flat face would only really make a difference to the performance to a bullet after in exits the barrel.
Fourth thing, there are places that have very powerful flash x-ray imagining capabilities where it would be possible to actually take x-rays of the chamber/rifling startup area of the barrel at several intervals in time as pressure is building and the round is starting to move. This make be a way to interrogate exactly what the bullet is doing as it engages the lands and begins its travel down the barrel in terms of rotation.
Specific Application question:
I want to build a rifle in 300 PRC. I want to mainly use it for long range hunting (spending a decent amount of time practicing on targets). I plan to use monolithic bullets such as Hammer, Barnes, Cutting Edge etc. My question is, can I use gain twist to help me stabilize some of the really long mono bullets but still be able to shoot the lighter ones? Hammer 152 grain needs 1-12, but 227 grain needs 1-7.5 twist. If they are all mono bullets from the same material is "over spinning" really even an issue? As in, if the 227 grain bullet can take the spin, shouldn't the 152 grain bullet from the same material be able to take the spin?
Second application question, is there diminishing returns here chasing the longer higher BC bullets because of the twist requirements? If you have to achieve the 1-7.5 twist for the higher BC bullet, and assuming you do it with gain twist, is there enough of a chance that you could have a bullet leave the barrel that is damaged from the aggressive twist that it's not worth it? The Hammer 199 grain only needs 1-9.25 twist, maybe doing a gain twist that finished at 8.75 and just ignoring those heaviest bullets is the best option?
I apologize that this post got so long!
And, I apologize in advance for my typos! (I'm ok at doing science, but I am pretty shit at writing it up and proofreading)
Yo,
General:
1. There’s enough engraving that a typical bullet doesn’t skip over any lands or anything. Soft lead bullets with no jackets that are spun very quickly can smear the rifling engravings off, but that’s a super niche thing that basically isn’t relevant today.
2. The parameter you’re talking about is “freebore” and is mostly related to chamber pressure for a given powder charge. Anecdotally, higher pressures and faster twists at the end of the freebore seem to correlate with damage to the jacket as signified by bullets exploding out of the muzzle when they otherwise shouldn’t.
3. I’m not aware of direct velocity measurements going down different barrel profiles; twist does not seem to matter at all for velocity. A flat faced bullet will be ballistically useless and different driving band lengths and different bullet weights and such will mean that the velocity from a given twist rate or rifling parameter for one bullet won’t be useful for comparing that performance for a different bullet. As such I’m not sure there’s much value doing this.
4. That would be interesting. X-ray scans through metal take time to get accuracy, though, so there are some technical details to work out if the data is to be useful at bullet timescales.
Application:
1. Overspinning increases dispersion on monolithics, but won’t blow them up. The listed twist rates are the minimum acceptable twists in most shooting environments, because a copper-alloy monolithic is longer and must therefore spin faster to be stable than an equivalent-weight lead core bullet. Pick the fastest twist rate for the bullet you’ll use; probably a 1:8 for the 212 Seneca projectile. I don’t think the 240-class monolithic projectiles are worth it in 300 PRC since the 200- and 210-classes have enough more velocity to have a better hit percentage at range.
2. Twist rate doesn’t damage monolithics, don’t worry about it, and there’s evidence that faster twist increases ballistic coefficient anyway (at the expense of dispersion due to gas handling at the muzzle). Gain twist doesn’t seem to be that useful for monolithic performance vs a straight twist at the desired muzzle twist rate.
Hi gnochi, thanks for answering. I have responded to your responses below. Mostly to agree
General:
1. Thank makes sense, if bullets were smearing over the grooves we would see it in the engraving.
2. This also makes sense, if you push it faster into a steeper angled lands area you are more likely to cause damage.
3. Agreed, you would need to use a bullet (or custom bullet) with the same bearing surface to be able to make valid comparisons. I know that a flat face bullet wouldn't perform ballistics wise, but in this case that wouldn't matter as long as the back end, lands engagement area. and bearing surface were the same, the out-of-barrel performance wouldn't matter for the test. Even though final velocity out of the end of the barrel doesn't change with twist rates, it might be interesting to see if the velocity vs time profile changes. I.E. we are imparting the same total momentum either way, but is the initial profile different.
4. You are absolutely right, if the duration of x-ray pulse required to get enough photons on the sensor or film is too long, then motion blur would degrade the image and make it pointless. As with highspeed video and fast moving objects (short exposure times are needed). The x-ray sources I am referring to are 18MeV + and can deliver a few hundred rad in a couple micro seconds. We regularly capture (without blur) objects in motion that are travelling at several thousand meters per second. All that being said, careful consideration would have to be made regarding the barrel specs in terms of steel thickness vs dose to ensure the number of photons delivered to the image plane after passing through the barrel and bullet was sufficient to ensure a decent signal to noise ratio. There are some modelling codes out there than are used to predict what an image through an object will look like based on its geometry, material properties, imaging system and the radiation source used. You would want to do that first to "see" what you would see before bothering with any actual experiments.
Application:
1. Makes a lot of sense.
2. I guess there is no harm in specing a gain twist if I don't mind the increased lead time, then if I do decide to shoot something other than monoliths I have it.
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It’s got less than 300 rounds on it and its looking like it may surpass my “go to” rifle.
The previous “Go to” at 300 yards with 175 FGMM…1:11.25 Krieger SS
4 inch target. Twenty rds 175 FGMM.
The new gain twist….some redonculous gain twist I came up with because I have no clue and figured I could pull some number out of my ass and claim myself a ballistic genius.
Deets are probably in here in an earlier post.
I did follow the Lowlight directive not to exceed .75 in GT. Originally I was thinking I could go from like 1:11.25 to something near a 1:9 and the bullet would act like a car moving through transmission from neutral to 6th gear.
I don’t think it works that way and I’d be shmeering jacket the length of the bore had I done do.
At 300 yards….
4” Circle….20 rds FGMM
3” Circle…..20 rds FGMM
At 525 yards….175 FGMM 10 or so rds.
End of day I think I shot 30-40 rounds and was the only one that could hit the plate.
Looking back to the shooting position.
Shooting position toward target….
The previous “Go to” at 300 yards with 175 FGMM…1:11.25 Krieger SS
4 inch target. Twenty rds 175 FGMM.
The new gain twist….some redonculous gain twist I came up with because I have no clue and figured I could pull some number out of my ass and claim myself a ballistic genius.
Deets are probably in here in an earlier post.
I did follow the Lowlight directive not to exceed .75 in GT. Originally I was thinking I could go from like 1:11.25 to something near a 1:9 and the bullet would act like a car moving through transmission from neutral to 6th gear.
I don’t think it works that way and I’d be shmeering jacket the length of the bore had I done do.
At 300 yards….
4” Circle….20 rds FGMM
3” Circle…..20 rds FGMM
At 525 yards….175 FGMM 10 or so rds.
End of day I think I shot 30-40 rounds and was the only one that could hit the plate.
Looking back to the shooting position.
Shooting position toward target….
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…..PS…..
It’s not only GT but it’s LH GT because of aereolas in the Northern Hemispheres.
It’s not only GT but it’s LH GT because of aereolas in the Northern Hemispheres.
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Not having one, it seems that it would work well on an overbore type cartridge like the 6 Creedmoor.
Than it would be RH GT and I’d measure the GT in marsupials.
Running a Valkyrie with a LHGT bbl. No complaints, and it tends to shoot most loads well.
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