Rifling Twist-Rate Effect on Chamber Pressure

[Jim Boatright]

Just saw a mention that some new gun shown at this year's SHOT Show comes with a 3-inch twist rifled barrel. Some barrel makers have made those kind of twist-rates for ballistic test barrels used for short-range, reduced velocity testing of subsonic bullet behavior or extreme range terminal ballistics. Some riflemen think fast twist barrels cause a problem with excess chamber pressures. Super fast-twist barrels DO cause some increase in inertial resistance during firing, but not nearly as much as most riflemen seem to imagine. Engraving and barrel friction forces are not increased with faster twist rifling, all else being equal. The formulation for the extra inertial resistance in accelerating the engraved bullet with faster rifling twist-rates is a bit tricky, but I thought about it awhile and believe I have it correct.

EDIT: I found the expression for friction-free torque versus clamping force for machine screw threads in Shigley's Mechanical Engineering Design (4th Ed.), page 365, equation (h), where the rifling twist (n*d) is substituted for thread pitch (1/TPI, for example). I have corrected my formulation accordingly.

With linear acceleration (A), the resistive force (F) is given by bullet mass (m) multiplied by that acceleration (A). Adding the "spin-up" torque due to rifling twist can be formulated as effectively equivalent to increasing the bullet mass (m) by a small fraction. That small fraction is 0.0113 (or 1.13-percent increase) for my 265-gr copper ULD bullet fired from a 6.6-inch twist 338-caliber barrel made by Bartlein. That is for a twist-rate of 20 calibers per turn, which is very quick twist. [I use the bore diameter 0.330-inch as 1.00 calibers.] That is to say, firing this 265-gr copper ULD bullet through a 6.6-inch twist barrel will produce the same pressure profile as firing a copper bullet weighing 268 grains from a hypothetical zero-twist rifled barrel.

If you want to find this fractional effective bullet mass (or weight) increase (f) for your rifled barrels, here is the formulation:

f = [(2 Pi/n)*(kx/d)]^2

where n = number of calibers (d) per turn for your rifling and kx is the Radius of Gyration of the mass distribution of your rifle bullet about its spin-axis (x). Any monolithic ULD or VLD rifle bullet will have kx/d (0<kx/d<0.5) very near to the 0.338 of my bullet designs. [I will happily share my development of this formulation with any who care for it.]

For a more typical twist-rate of n = 40 calibers per turn, say a 30-caliber barrel with a 12-inch twist-rate, the effective bullet weight increase would be just one quarter as much (0.00282), and so forth. About this much effective bullet weight increase (0.75 grains here) is already factored into QuickLOAD(c) interior ballistics calculations because they do not even ask for input of rifling twist-rate, apparently assuming it will be in this "normal" range. So, one could argue for reducing the formulation shown above by an additional factor of 0.75.

I don't buy any argument for selecting a gain-twist barrel to prevent early rise in chamber pressures. The pressure difference is too small to risk twisting your rifling engraved bullets as they traverse the bore of any gain-twist barrel.

 

[Steel head]

That’s pretty interesting concept that I actually understood this time.
Cool that’s it’s showing effective bullet grains weight by twist and how minute the change is.

 

[nick338]

risk twisting your rifling engraved bullets as they traverse the bore of any gain-twist barrel.

Can you elaborate on or provide a link to something you've probably already written about this?

 

[nick338]

The gain twist concept has always bugged me. If the rate of twist is not constant for the length of the bore, wouldn't that mean the same rate is also not constant from the initial engraving of the projectile until it leaves the barrel, imparting a different force on it?

 

[Jim Boatright]

I have never written seriously about gain-twist rifle barrels. My good friend and mentor, M. L. (Mic) McPherson of Colorado, told me privately that the recovered hard cast 50-caliber bullets they fired at one mile from Sharps rifles showed them to have been twisted by gain-twist rather than having their engraving marks widened. IIRC, they were recreating the 1538-yard Billy Dixon shot which ended the Battle of Adobe Walls in the post Civil War Texas Panhandle. My rear driving band copper bullet designs would not suffer twisting in gain-twist barrels, but most other bullet types would. I just do not see enough possible advantages to warrant messing with them. Super fast-twist conventional rifling designed for gas obturation works perfectly well.

 

[nick338]

Makes sense, thank you.

 

[Jim Boatright]

I corrected my OP to bring my derived expression for the torque/force relationship into agreement with Shigley. I mentioned that this question was kind of tricky. The rifling twist effects on chamber pressures are even smaller than I had first thought (0.85-percent increase in effective bullet weight for a very fast 20 calibers per turn twist-rate).

 

[nick338]

I would think the jamming a bullet into the rifling would have more of an effect on chamber pressure and the actual twist rate would be negligible. Consequently, a heavy bullet and faster twist may translate into more felt torque for the shooter and induce jacket/core stress as the projectile enters the rifling. So if I'm understanding it correctly, the gain twist is used to mitigate this but I feel at the expense of actually twisting the bullet due to the external forces imposed by the variation in twist/increase rpm as it nears the muzzle, the more aggressive the gain exacerbating this.

 

[Steel head]

I corrected my OP to bring my derived expression for the torque/force relationship into agreement with Shigley. I mentioned that this question was kind of tricky. The rifling twist effects on chamber pressures are even smaller than I had first thought (0.85-percent increase in effective bullet weight for a very fast 20 calibers per turn twist-rate).

Do you have any suggestions for reading on internal ballistics for the jethro clampetts of the world like me?

 

[Jim Boatright]

I would think the jamming a bullet into the rifling would have more of an effect on chamber pressure and the actual twist rate would be negligible. Consequently, a heavy bullet and faster twist may translate into more felt torque for the shooter and induce jacket/core stress as the projectile enters the rifling. So if I'm understanding it correctly, the gain twist is used to mitigate this but I feel at the expense of actually twisting the bullet due to the external forces imposed by the variation in twist/increase rpm as it nears the muzzle, the more aggressive the gain exacerbating this.

Yes, Shot-Start pressure (required bullet engraving pressure) is seriously affected by throat angle and smoothness, bullet jump (if any), bullet hardness and possibly by anti-friction surface coatings. It is almost independent of rifling twist-rate selection. Higher SS pressure is deleterious in that it increases peak chamber pressure more than it increases muzzle speed of the fired bullet. Jam seating is used in short-range benchrest competition to improve consistency of the peak chamber pressures from shot to shot for more consistent bullet exit times and muzzle speeds.

Felt torque reaction increases linearly with increases in peak chamber pressure, in caliber, and in bullet mass. It could be mitigated by using a muzzle attachment producing a torque reaction via gas momentum recapture which acts in the same sense as the bullet's spin.

I don't like to fire bullets where core stripping or rifling skid marks might be issues. Bonded cores of stronger core alloys and using more work-hardened gilding metal jackets can alleviate these problems in the making of jacketed bullets. These are good reasons for instead selecting monolithic copper rifle bullets made from cold-rolled (HO2 half hard) copper rod stock. Since peak base-pressure, and consequently peak spin-up torque, occurs after just a few inches of bullet travel, gain-twist rifling can ameliorate these bullet integrity problems by 20-percent or so at that end of the barrel, only to have the bullet twisted by that same percent increase in the helix angle of the rifling farther down the barrel.

 

[Jim Boatright]

Do you have any suggestions for reading on internal ballistics for the jethro clampetts of the world like me?

Not much is written concerning interior ballistics except for obtuse military engineering studies. I would suggest (1) using QuickLOAD(c) interior ballistics software for load development and (2) recovering bullets fired (at >45-degree angle) into the deep end of a swimming pool for visual studies. [Wear rain-gear and hearing protection and have kids dive to recover the bullets.] Much can be seen or inferred about bullet performance by using these approaches.

 

[JB.IC]

I always thought gain twist rifling would smear the surface of a bullet and cause variations in the bullets BC

 

[Jim Boatright]

I always thought gain twist rifling would smear the surface of a bullet and cause variations in the bullets BC

I thought so too, JB, until I talked with Mic McPherson, who actually used them with lead bullets and black powder--and then recovered some of the fired bullets. He said that the engraving marks were clean and just one land width and with the helix angle of the final twist-rate. I have no experience with gain-twist barrels myself. That is a complication I have been able to do without. IIRC, Mic also said that jacketed bullets also twisted when fired from gain-twist barrels.

 

[John Glidewell]

Not much is written concerning interior ballistics except for obtuse military engineering studies. I would suggest (1) using QuickLOAD(c) interior ballistics software for load development and (2) recovering bullets fired (at >45-degree angle) into the deep end of a swimming pool for visual studies. [Wear rain-gear and hearing protection and have kids dive to recover the bullets.] Much can be seen or inferred about bullet performance by using these approaches.

very interesting stuff. I use quickload and run on barrel times. Usually the 4.5 node is about 1.5 grains back from max and that seems to be where they run the best. I have run them at the next node up but it is vey close to max and seems to be a bit of a scatter node, the 1 in 5 flier phenomenon. I started doing this after talking with a bunch of folks at shot show in 2020, Bryan Litz and a bunch of the king of 2 mile shooters about load development. Quick load just makes it easier / faster. Find the barrel time and shoot across it, one of them will shine. I am also running a gain twist on one of my rifles right now. I was worried it would skew the QL data but it does not appear to have any negative impact on the data. But people should be warned that QL is a BS in BS out type data system. It took abit of learning/tinkering but once I figured it out I can build a profile for a rifle pretty fast and it will reflect actual performance.

 

[Baron23]

The pressure difference is too small to risk twisting your rifling engraved bullets as they traverse the bore of any gain-twist barrel.

Do you have any sort of quantification of this risk? It seems like this is kind of thrown out there without any supporting data...well, unless I missed that in reading the thread which is quite possible.

Cheers

 

[Jim Boatright]

Do you have any sort of quantification of this risk? It seems like this is kind of thrown out there without any supporting data...well, unless I missed that in reading the thread which is quite possible.

Cheers

I have no data on hand showing bullet twist upon firing from a gain-twist barrel, but I trust the observations of Mic McPherson, cited above. It also makes sense mechanically to me. The differential torque applied to the bullet shank by the rifling would be enormous in traversing a GT barrel. The "grip" of the engraved rifling lands upon the bullet shank would also be very strong. The shank of a monolithic copper bullet would spring back elastically upon muzzle exit (at least partially), but the rifling engraving marks on the recovered bullets would still be clean straight lines, perhaps at some intermediate helix angle between ArcTan(Pi/n1) and ArcTan(Pi/n2), where n1 and n2 are the initial and final twist-rates, respectively, in calibers per turn.

 

[Baron23]

I have no data on hand showing bullet twist upon firing from a gain-twist barrel, but I trust the observations of Mic McPherson, cited above. It also makes sense mechanically to me. The differential torque applied to the bullet shank by the rifling would be enormous in traversing a GT barrel. The "grip" of the engraved rifling lands upon the bullet shank would also be very strong. The shank of a monolithic copper bullet would spring back elastically upon muzzle exit (at least partially), but the rifling engraving marks on the recovered bullets would still be clean straight lines, perhaps at some intermediate helix angle between ArcTan(Pi/n1) and ArcTan(Pi/n2), where n1 and n2 are the initial and final twist-rates, respectively, in calibers per turn.

Thank you for your reply.....I would suppose this "risk" would be highly dependent on the degree of gain...starting twist and ending twist rate. Bartlein recommended a .5 to .75 degree of gain for a .264 blank when I raised the topic with them.

@Frank Green - have you had any reports of cup and core bullets deforming (twisting) when shot out of your gain twist barrels.

Truth in advertising, I do have a 8.25-7.5 gain twist Bartlein barrel being chambered...well, kind of right now and I'm looking forward to getting it.

Cheers

 

[Jim Boatright]

This thread was really about showing riflemen how to quantify the peak chamber pressure increase which would be caused by selecting a faster rifling twist for a new barrel. Just multiply your expected bullet weight by a factor of {1 + [(2 Pi/n)*(kx/d)]^2} and run that bullet weight in QuickLOAD.

Yes, Gwain, QL is an interior ballistics simulator program, and as with a 6 DoF exterior ballistics simulator, inaccurate program initialization data badly skews the calculated results into something unrealistic. Garbage in, garbage out, or "GIGO" as we used to say in the engineering profession.

 

[Frank Green]

Thank you for your reply.....I would suppose this "risk" would be highly dependent on the degree of gain...starting twist and ending twist rate. Bartlein recommended a .5 to .75 degree of gain for a .264 blank when I raised the topic with them.

@Frank Green - have you had any reports of cup and core bullets deforming (twisting) when shot out of your gain twist barrels.

Truth in advertising, I do have a 8.25-7.5 gain twist Bartlein barrel being chambered...well, kind of right now and I'm looking forward to getting it.

Cheers

I haven't heard of a single failure of bullets per say and it being directly related to gain twist of any sort.

We even made ammunition pressure test barrels in 308win. and 6.5CM and the twist went from 12 at the breech to ending at 8 twist. Barrels ran like a watch with no issues etc...

Nearly all of the top BR shooters running our barrels on 6PPC guns are getting a gain twist in some form.

Out of all the guys here in the shop that have a GT barrel on a gun regardless of caliber....again haven't seen a issue at all.

So again I will say no on the bullet failure thing and a gain twist barrel.

Later, Frank
Bartlein Barrels

 

[Geno C.]

@oneshot.onehit has a recovered bullet from a pretty extreme gain twist barrel. If he posts the pic, you’ll see it’s not as much deformation as most think

 

[oneshot.onehit]

@oneshot.onehit has a recovered bullet from a pretty extreme gain twist barrel. If he posts the pic, you’ll see it’s not as much deformation as most think

 

[JB.IC]

Wonder what the doppler radar reads from a standard twist verses a gain twist. Would be interesting to see some data of a statistically valid sample

 

[Steel head]

View attachment 7793567

Buddy is trying to make the 338 version of that barrel work with modern solids.

So far kinda meh.

 

[Jim Boatright]

Thanks for the information guys. I never said anything about "bullet damage" due to gain-twist barrels.

If I am reading the barrel stamps correctly, the rifling of this 6-groove .366/.375 barrel starts at 16-inch/turn twist-rate an increases to 7.78-inch/turn near the muzzle. The rifling helix angle would have increased from an initial 4.21-degrees to a final 8.61-degrees as the engraved bullet traversed the barrel in firing. The rifling marks on the recovered copper bullet suggest several things to me. The helix angle on the left (non-driving) side of the facing land engraving mark appears to be at the initial 4.2-degree helix angle. The copper displaced during original engraving of the driving rings remains only along the non-driving side, and mostly toward the front of the bullet. The rear-most driving ring is the area of the copper bullet where the contact pressure against the bore was greatest during firing. As the land driving the spin-up of the bullet rotated clockwise with increasing helix angle, the marks indicates that it "pivoted" about the rear-most ring position. The left-ward side pressure of that pivoting land wore away the trailing copper displacement marks there, just as all of those marks are gone from the right-hand driving side of this top-side engraving mark. The engraved width of the rifling mark also more closely matches the narrow land toward the rear of the bullet. The front-most end of the engraving mark is significantly wider than the narrow land itself. I cannot read the helix angle along the right-hand side of the mark due to wear, but it seems to be less than the full final 8.6 degrees--indicating that most of the twisting of the copper shank was elastic and it simply sprung back as the bullet was exiting the muzzle.

This extreme gain-twist example is great for studying its effect on fired bullets. This copper bullet survived quite well. Thanks again for the information.

 

[Lowlight]

i have at least 8 Left Hand Gain Twist Barrels and they are better than any other barrel of equal quality out there

They shoot better, reload easier, don’t care about bullet weight, the Gain Twist Barrels WORK !

Science is observation right, well my many observed barrels have shown me that I order gain twist barrels whenever possible

just dont tell the computer programs they work, they wouldn’t under it

 

[Macht]

There are additional reasons for the use of gain twist. A large percentage of large (>20mm) rifled guns use gain twist in order to reduce the rotational acceleration seen by the projectile. This is important for instrumented projectiles, and becomes increasingly critical as the forcing cone wears. As erosion proceeds, the axial velocity of the projectile when first encountering the rifling also increases, thereby increasing initial rotational acceleration. A gain twist reduces this effect while still achieving the final twist needed to stabilize the projectile.

 

[Jim Boatright]

Starting the engraved bullet into rifling having a steeper helix angle DOES increase the peak chamber pressure a little, but not nearly as much as many seem to think. I just wanted to quantify that increase so you can make more informed decisions.

I am neither for nor against using gain-twist barrels. Just don't tell me I should do so to prevent some large increase in chamber pressure with fast-twist barrels.

 

[JB.IC]

Starting the engraved bullet into rifling having a steeper helix angle DOES increase the peak chamber pressure a little, but not nearly as much as many seem to think. I just wanted to quantify that increase so you can make more informed decisions.

I am neither for nor against using gain-twist barrels. Just don't tell me I should do so to reduce chamber pressures with fast-twist barrels.

i appreciate your feedback and the research you share with us.

 

[Frank Green]

Buddy is trying to make the 338 version of that barrel work with modern solids.

So far kinda meh.

Solids are a whole animal unto themselves at times.

One thing with the solids and this is my opinion....again my opinion and I'm not a bullet maker but the solid bullets don't give like a conventional lead core/jacket type bullet. I feel the main diameter of the bullet should be just slightly over groove size to get them to shoot well. I'm not saying .005" but something like around .0015" bigger than actual groove size of the barrel. Another bullet maker has expressed the same thing to me.

So for one customer solid bullet maker we do stamp the breech end of the barrel to the 4th decimal place per so they know the actual groove dimension on that given barrel. Then when they make bullets for that customers gun etc...they run the bullets to a given size + for that particular barrel.

Other things that are different are bullets with driving bands and with rebated bands etc...which is better? Does it matter?

Don't even start the topic on fouling either...another animal when it comes to solids in my opinion.

 

[JB.IC]

The driving band aligns with Jim’s research on bullet obturation. In his paper he discusses how jacket bullets obturate better than solids bullets and that is one reason why a driving band is important on solids.

It’s a very good paper and with all of his research (if you can understand the math parts it’s even better. I’m not that person lol)

 

[Jim Boatright]

Here is a PDF of a paper I just wrote detailing the derivation of this formulation for effective bullet weight increase when fast-twist rifle barrels are used. The bullet weight increase is only 0.959-percent when a very fast 20 calibers per turn barrel is used such as my 6.6-inch twist Bartlein 338-caliber barrel.

 

[Steel head]

Solids are a whole animal unto themselves at times.

One thing with the solids and this is my opinion....again my opinion and I'm not a bullet maker but the solid bullets don't give like a conventional lead core/jacket type bullet. I feel the main diameter of the bullet should be just slightly over groove size to get them to shoot well. I'm not saying .005" but something like around .0015" bigger than actual groove size of the barrel. Another bullet maker has expressed the same thing to me.

So for one customer solid bullet maker we do stamp the breech end of the barrel to the 4th decimal place per so they know the actual groove dimension on that given barrel. Then when they make bullets for that customers gun etc...they run the bullets to a given size + for that particular barrel.

Other things that are different are bullets with driving bands and with rebated bands etc...which is better? Does it matter?

Don't even start the topic on fouling either...another animal when it comes to solids in my opinion.

That barrel was designed years ago for a banded solids project and guessing from his results it’s a bit too tight.

I’m running the same solids in one of your 338 1-8 barrels now with excellent results with another baron order.

 

[Lowlight]

I have a 13-5 twist 338 Norma barrel that was produced for a solid project several years ago

Starts at 13, ends at 5

 

[Lowlight]

The two solids at the bottom were designed for that 13-5 twist barrel

 

[Geno C.]

.0015” over might be a bit much but better to seal than not. I know there are barrels out there that are not shooting some bullets to their potential because the bullets were made too close to groove size and there’s a little blow by

 

[Jim Boatright]

Solids are a whole animal unto themselves at times.

One thing with the solids and this is my opinion....again my opinion and I'm not a bullet maker but the solid bullets don't give like a conventional lead core/jacket type bullet. I feel the main diameter of the bullet should be just slightly over groove size to get them to shoot well. I'm not saying .005" but something like around .0015" bigger than actual groove size of the barrel. Another bullet maker has expressed the same thing to me.

So for one customer solid bullet maker we do stamp the breech end of the barrel to the 4th decimal place per so they know the actual groove dimension on that given barrel. Then when they make bullets for that customers gun etc...they run the bullets to a given size + for that particular barrel.

Other things that are different are bullets with driving bands and with rebated bands etc...which is better? Does it matter?

Don't even start the topic on fouling either...another animal when it comes to solids in my opinion.

Yes, Frank, the concerns you mention (along with several others that we have discovered) are valid for monolithic copper bullets, but there are ways around all of them, and the results are worth making the efforts.

I have developed and patented a turned copper ULD bullet design which offers several unique advantages. Perhaps most importantly, air drag is 30-percent less at Mach 2.5 than that of the G7 Reference Projectile. I am not going to manufacture or market them, but have placed the US patents into the public domain. Others can freely make and sell them, but they cannot patent them again.

Dan Warner has been turning them for me in 338 caliber from 0.5-inch cold-rolled (H02) UNS C147 copper rod stock. The bullets are 6.0 calibers long, with a 3.5 caliber LD Haack head shape connected to a 1.00-caliber shank diameter. At 0.3302 inches, the 1.3-calibers long bore-riding shank will freely enter the rifling lands beyond the chamber throat during insertion of the 338 Lapua Magnum cartridge. COAL is 4.25 inches in 338LM. The finished weight of the solid copper 338 bullets is 265 grains. After the 1.3 caliber bore-riding shank, there is a 0.1 caliber 7.5 degree transition cone up to a rear driving band of 0.3382-inch diameter. The top length of the RDB is 0.4 calibers, and it connects directly to a 7.5 degree conical boat-tail of 0.7 calibers in length. The nose is machined sharp and the 7.5 degree inside and outside corners of the transition cone are radiused at 0.150-inch. The join of the boat-tail to the RDB is also radiused at 0.150-inch, but the end of the BT is machined sharp. The aerodynamic shape is very clean.

The center of the 0.842 caliber BT base is drilled with a 0.45 caliber (0.152-inch) drill to a tip depth passing inside the rear 0.3 calibers (3/4) of the RDB. By porting the base-pressure inside the RDB, very good gas sealing is obtained, resulting in minimum copper fouling (after polishing the <1.5-degree throat) and often achieving 5-shot extreme spreads in MV of 2 to 4 fps. A short ball seat of 0.3386 inch ID minimizes blow-by just after bullet release. The front 1/3 of the RDB must enter the ball seat as the cartridge is being loaded to prevent copper shaving as the bullet releases from the neck tension during firing. The dual-diameter bore-rider design completely eliminates the in-bore yaw during rifling engravement which plagues all other bullet designs. It also contributes 5 to 6-percent to the drag reduction by reducing the base area of the LD Haack headshape. The turned monolithic copper bullets are inherently statically and dynamically balanced which eliminates the drive toward slowest rifling twists needed to minimize lateral throw-off just out of the muzzle (as with all jacketed bullets).

For optimum gyroscopic and dynamic stability (damping of coning angles) in flight, these bullets deserve to be fired from barrels having rifling twist-rates of 20 to 24 calibers per turn. A minimum land width rifling pattern with good gas-sealing groove design (with radiused inside corners) is also beneficial. For best accuracy and lowest early flight air-drag, these long copper bullets must exit the muzzle at a time of near zero lateral muzzle acceleration, not just minimum lateral velocity. This is a new type of barrel-to-load tuning which is more easily obtained with short barrel lengths or by using barrel-block rifle construction with long barrels. I have written an Excel spreadsheet for calculating muzzle motions for all possible bullet exit times from barrel and muzzle attachment specifications and QuickLOAD interior ballistics outputs. The spreadsheet is freely available as an email attachment <[email protected]>.

Jim Boatright

 

[Jim Boatright]

.0015” over might be a bit much but better to seal than not. I know there are barrels out there that are not shooting some bullets to their potential because the bullets were made too close to groove size and there’s a little blow by

I tried 0.0006-inch and 0.0004-inch over-diameter solid copper rear driving bands (having no drag producing "relief grooves") and found them to be too large (12 and 10 ksi Shot-Start pressures, respectively). I went to a 0.0002-inch oversize RDB and solved the copper bullet engraving problem using a polished 1.5-degree throat angle. [a just >0.5-degree throat angle might be better.] My rear driving bands have a solid top-width of 0.4 calibers for gripping in the case neck and entering the ball seat during cartridge loading into the rifle. Base-drilling through the boat-tail and under the RDB is REQUIRED to get perfect gas sealing (Barrel obturation at peak base-pressure) with any copper bullet design.

 

[Jim Boatright]

There are additional reasons for the use of gain twist. A large percentage of large (>20mm) rifled guns use gain twist in order to reduce the rotational acceleration seen by the projectile. This is important for instrumented projectiles, and becomes increasingly critical as the forcing cone wears. As erosion proceeds, the axial velocity of the projectile when first encountering the rifling also increases, thereby increasing initial rotational acceleration. A gain twist reduces this effect while still achieving the final twist needed to stabilize the projectile.

Yes, compared to rifle bullets, most large military gun projectiles are of much lower average density, lower scaled weight, lower scaled strength of construction, and even often lower sectional densities. [Liquid payloads must be particularly difficult.] That is why I stick to flat-firing of rifles 20 mm or smaller.

 

[oneshot.onehit]

Just for the record the pictured barrel cutoff and bullet. The barrel is being used in a 375 Snipetac that is shooting the Warner 361s and the Badlands very well in the ELR game currently lately.

osoh

 

[Frank Green]

.0015” over might be a bit much but better to seal than not. I know there are barrels out there that are not shooting some bullets to their potential because the bullets were made too close to groove size and there’s a little blow by

I had butter fingers and brain fade...meant to type .00015". So say .0001" to .0002" over actual groove size of the barrel.

 

[Steel head]

I had butter fingers and brain fade...meant to type .00015". So say .0001" to .0002" over actual groove size of the barrel.

This combination I’m running now has been pretty good
Bartlein 1-8 5r stamped .330 and .338 and 265 solids that are .3383

 

[Geno C.]

I had butter fingers and brain fade...meant to type .00015". So say .0001" to .0002" over actual groove size of the barrel.

Makes more sense lol. That’s a little problem with some bullet makes because are a barrel company, you’re shooting for nominal +.0005 as a tolerance. If a guy is trying to shoot for, say, .3082” and gets a barrel that’s .3084” then it will never shoot great. You guys offering the 4th place measure to the consumer will help avoid that issue. I think that’s why cutting edge bullets work in a broader range of barrel. Their seal tight band takes up that tolerance without needing to be as accurate with the diameter match.

 

[Jim Boatright]

Makes more sense lol. That’s a little problem with some bullet makes because are a barrel company, you’re shooting for nominal +.0005 as a tolerance. If a guy is trying to shoot for, say, .3082” and gets a barrel that’s .3084” then it will never shoot great. You guys offering the 4th place measure to the consumer will help avoid that issue. I think that’s why cutting edge bullets work in a broader range of barrel. Their seal tight band takes up that tolerance without needing to be as accurate with the diameter match.

With skill and care, I believe the CNC turning of "free machining" (C145 or C147) half-hard copper bullets can be done maintaining +/- 0.0001-inch tolerances in turning diameters. The bullet makers do this primarily to maintain consistent bullet weights. Everyone seems to weigh their expensive copper bullets as a quality check. I use a lab balance. I specify the OD of the RDB to be 0.3382-inch +/-0.0001 for my 338-caliber copper ULD bullets.

If the barrel groove ID is maintained to +/-0.0005-inch of nominal, that is fine, especially if measured and stamped to four decimal places after lapping. The shank diameter (ID) of 0.3302-inch +/-0.0001 is not a problem for the bore-riding bullets. The lands are fire-lapped as part of polishing the throat with David Tubb's Throat Maintenance System (fine abrasive impregnated jacketed bullets). [Download them or they will disintegrate out of the muzzle with fast-twist barrels.]

My idea of base drilling the monolithic copper bullets allows adequate gas sealing even in worn over-diameter barrels. The barrel ID expands by almost 50-percent more than most believe at peak base-pressure, regardless of barrel steel selected or OD at this location. Without base-drilling, no copper bullet can expand this much and maintain a gas seal. At the cost of 10 grains in finished bullet weight, you get minimum copper fouling (none after break-in), single-digit muzzle speeds (in fps), and flexibility in handling various bore ID's. The lighter bullets start faster anyway. With their ultra-low-air-drag, they could remain supersonic to about 2 miles for the big boomers.

Launching the copper ULD bullets without imparting any accuracy robbing and energy sapping tumbling motion upon exit from the muzzle remains the single largest hurdle for riflemen to solve. That is why I spent 4 months writing that muzzle motion calculating Excel spreadsheet. As I have said before, just email me at <[email protected]> and I will send it as an attachment to my reply, along with PDF's of a couple of explanatory papers.

Jim Boatright

 

[KZP]

I have a 13-5 twist 338 Norma barrel that was produced for a solid project several years ago

Starts at 13, ends at 5

Are there any rules of thumb for determining the gain twist start and end? Is it a linear progression to reach the end twist?

 

[Lowlight]

today most of us are doing a 3/4 gain

So for a 6.5 I do a 8.25 to 7.5

But different groups are using different gains, the 556 groups are doing 14 -6 as I have read, and I think the Benchrest guys are 1/2 gain, so like 8.5 to 8 would be an example

 

[Frank Green]

Makes more sense lol. That’s a little problem with some bullet makes because are a barrel company, you’re shooting for nominal +.0005 as a tolerance. If a guy is trying to shoot for, say, .3082” and gets a barrel that’s .3084” then it will never shoot great. You guys offering the 4th place measure to the consumer will help avoid that issue. I think that’s why cutting edge bullets work in a broader range of barrel. Their seal tight band takes up that tolerance without needing to be as accurate with the diameter match.

add to this....now you get a guy buying this or that custom solid bullets...and they run it thru a factory barrel where the bore sizes vary up to .002". Seen it plenty of times. Just magnifies any issues.

Three different bullet makers have told me the bore/groove size of the barrel is more important with the solids. They are more sensitive to it.

Also as has been said in a earlier post... (think Jim B said it) the solids don't obturate like a conventional jacketed/lead core bullet does.

 

[jasent]

I don’t see a point in gain twist barrels. So they shoot well. So do none gain twist barrels. I always go faster twist than needed. Never had a negative doing so.

 

[Lowlight]

I don't get it...

LOL

 

[Jim Boatright]

Here is a PDF of a "revised and extended" version of this paper which should be more readily understandable.
EDIT: I updated the PDF to the published version on 3-1-22.

 

[Jim Boatright]

I have revised the original paper to make it more readable and added a table showing typical results as calculated by QuickLOAD(c). Both the chamber pressure increases calculated for faster twist-rate barrels and the associated slight losses in muzzle velocity are practically negligible until the twist-rate selected approaches an extreme of 10 calibers per turn. A PDF of the revised paper is attached.