Groove #, twist rates and velocity questions...

[Fred @ Bison Tactical]

Ok, I have some questions for people who are experts in this stuff (since I am not).

I hear a lot of people talk - in person and on the internet...

All other things being the same - is there a useful difference in velocity between say 3 groove barrels and say 6 groove barrels. The claim I have heard is there is some higher velocity to be gained going with less number of grooves vs more.

Same with twist rate. As long as I am stabilizing the bullet is a slower twist going to result in a higher velocity?

The main question being - is there a benefit of running a less common barrel configuration like say slower twist and less grooves to achieve a higher velocity with the same load?

@Frank Green - I am hoping someone who is an expert in this can chime in...

 

[cjs88]

Well whatever you read on the internet is probably 100% true

I'm no expert either, just a casual shooter, I've never heard of this. Would be interesting to see what the consensus is. If less grooves increase velocity/twist, why do more than 3?

 

[Fred @ Bison Tactical]

I think the idea is that there is some point at which there is less bearing surface and that means velocity can be higher, but obviously if you reduce the bearing surface too much then that would be "bad". That is why I am hoping to hear from some experts on barrel design for calibers mostly in the normal centerfire world.

 

[PinesAndProjectiles]

Polygonal rifling will generally provide an increase in velocity over a standard cut rifled barrel, and poly barrels typically have less bearing surface between the bullet and barrel.

So, to me it would make sense that less grooves = less bearing surface = more velocity.

 

[OREGUN]

Pretty sure the less twist = higher velocity thing has been disproven….I thought I read that here…from the barrel expert named above.

Not sure about rifling styles.

 

[Supersubes]

I think all of it is lost in the noise. The difference between two identical barrels, and the hand loaders feeding them, make far more difference. Fwiw, most makers try to construct bores that are roughly 70% groove and 30% land, regardless of number of grooves. I think this is why you dont see wildly different load requirements between configurations. I shot lilja 3 groove barrels quite a bit, and never thought they were fast. It was said that broughton 5c and Schneider 5P’s were fast as well, but i never saw it, and ran a few of each.

 

[Macht]

Litz covers the velocity/twist rate relationship in Volume 2 of Modern Advancements in Long Range Shooting. He discusses the theoretical basis for it (splitting a fixed amount of energy between linear velocity and rotational velocity means that a faster twist does result in a lower linear velocity) before presenting live fire results. I don't have it in front of me right now, but I believe the math indicated a very small difference and the live fire testing subsequently showed that it was essentially in the noise. (I'll edit this once I have a chance to check the book again.)

ETA: I got them mixed up, this was all in Volume 1. Both live fire and calculation showed an insignificant effect on velocity (single digit reductions for the tested twists).

 

[Supersubes]

Litz covers the velocity/twist rate relationship in Volume 2 of Modern Advancements in Long Range Shooting. He discusses the theoretical basis for it (splitting a fixed amount of energy between linear velocity and rotational velocity means that a faster twist does result in a lower linear velocity) before presenting live fire results. I don't have it in front of me right now, but I believe the math indicated a very small difference and the live fire testing subsequently showed that it was essentially in the noise. (I'll edit this once I have a chance to check the book again.)

I think the difference between the 8 and 12 twist was like 5-8 fps, but its been a while since I read it.

 

[spife7980]

. Fwiw, most makers try to construct bores that are roughly 70% groove and 30% land, regardless of number of grooves. I think this is why you dont see wildly different load requirements between configurations.

That was my understanding, when there are fewer of them, they just make them wider.

Too much engraving surface area and you get too much friction, too little and not enough.

 

[AccuSol-ERN]

Yeah I think the hocus pocus is in the land/groove ratio. For me, rifling configuration (number and shape) choice has more to do with my ability to accurately and consistently machine it for top accuracy.

Ern

 

[Fred @ Bison Tactical]

I think the difference between the 8 and 12 twist was like 5-8 fps, but its been a while since I read it.

Thanks for that - I read that, but its been so long ago I forgot about the article !

I think most of the issues anecdotally are when people get a particular barrel which shoots well for them, and they can't figure out why, so it must be the configuration of that particular barrel - without any real evidence behind the claims.

 

[Frank Green]

Ok, I have some questions for people who are experts in this stuff (since I am not).

I hear a lot of people talk - in person and on the internet...

All other things being the same - is there a useful difference in velocity between say 3 groove barrels and say 6 groove barrels. The claim I have heard is there is some higher velocity to be gained going with less number of grooves vs more.

Same with twist rate. As long as I am stabilizing the bullet is a slower twist going to result in a higher velocity?

The main question being - is there a benefit of running a less common barrel configuration like say slower twist and less grooves to achieve a higher velocity with the same load?

@Frank Green - I am hoping someone who is an expert in this can chime in...

I call bullshit! In a nice way! This has been beating to death.......

The style and number of grooves has no bearing on velocity and pressure.

We've made ammunition pressure test barrels in you name it caliber wise....308w, 6.5CM, 300WM, 338 Lapua, 300 Norma, 243W, 7mm Rem. Mag. etc...where the only difference was the number of grooves. Same chamber reamer was used. The bore and grooves made to the same spec/tolearnce etc....and we've done 3 groove, 4 groove, 5 groove, 5R, and 6 groove and 7 groove and 7R and 8 groove. The key is this phrase right here....The bore and grooves made to the same spec/tolearnce. If the bore and groove are being held to the same spec and tolerance and your running the same ratio for the groove width (total bore surface area) we see no difference in pressure and velocities.

We've made them for Hornady, Federal Cartridge, Lake City, etc...etc...etc...and they can find no difference from all the data that has been shared with us.

Later, Frank
Bartlein Barrels

 

[Frank Green]

Polygonal rifling will generally provide an increase in velocity over a standard cut rifled barrel, and poly barrels typically have less bearing surface between the bullet and barrel.

So, to me it would make sense that less grooves = less bearing surface = more velocity.

I'll disagree.

We've made polygonal rifle pressure test barrels for a ammunition and bullet maker. They wanted to test different rifling profiles etc...mostly coming at it from a standpoint of accuracy and how it effects long range accuracy on the bullet but they did pressure testing as well.

This was done in this case in 300WM and the polygonal rifling did not produce any higher velocities. So after that test they abandoned pursuing any more testing with polygonal rifling because they actually seen a little drop in velocity and that they didn't want to see at all.

If you look at the total surface area of the bore and you run the same dimensions/tolerances in the bore and groove in a barrel (also regardless of twist) and your using the same chamber reamer spec...you shouldn't see any significant difference in pressures and velocities.

So when a someone tells you this or that makes the barrel faster....you/they better know the details of the barrel. You should also know the bore and groove size to the 4th decimal place. If you don't know the dimensions...now your guessing as to what is going on.

Are the barrels made to the same bore and groove spec dimension wise?

Is the ratio of the groove widths the same?

Is the same chamber reamer spec being used in all of the barrels?

Not to mention ammo being shot thru it. You change any of the above spec...you can effect velocities and pressures as well as bullet diameters will effect things. Here are a couple of examples....

Changing the groove size has a bigger impact in pressures and velocities. I've seen pressure test barrels where the the bore and groove both where -.0005" undersize vs. the min spec. + the tolerance of .0005". That .0005" undersize dimensions raised the pressures 8000 to 10,000psi and drove up the velocities around 150fps. to almost 200fps. Same 6 groove barrels etc...just the one barrels where .0005" undersize on both the bore and groove.

Look at the chamber now...you start changing the throats freebore diameter and or length you can effect pressures and velocities as well. I seen on some 300AAC Blackout test barrels that we made....where the reamer cut the throat only .0001" to maybe .0002" undersize! Depending on which bullet you ran thru the barrels (some rocket scientist spec'd the bullet could be as large as .3090" diameter) well the throat is spec'd at .3090" diameter. So when they ran the bullets that where at .3090" diameter in the barrels (which is size on size with what the reamer is suppose to cut the chamber to and is the start of having problems) and in this case the reamer cut the throat to .3088"/.3089". It bumped pressures up 6k psi and also drove up the velocities. We got the barrels back and got a new reamer and just recut the chambers by hand....all of the barrels pressures and velocities dropped right into spec. That little difference of .0001" or so is all that it took to change things.

So it's more about uniformity and tolerances and dimensions and it's less about the style and number of grooves from everything I've seen.

So one of the things you will hear me say...."the straighter the barrel blank, the more uniform the bore and groove dimensions are over the length of the barrel, the more uniform the twist and the more stress free the barrel blank is....the more forgiving the barrel is going to be."

Later, Frank
Bartlein Barrels

 

[Frank Green]

I do believe in that the odd number of grooves like the 5R rifling (this doesn't apply to 3 groove barrels I will say) is that the 5R rifling does help fight bullet failure. This is because the odd number of lands will not directly oppose one another vs. a even groove barrel or a 3 groove barrel that has really wide lands.

When the lands directly oppose one another it will distort/upset the bullet jacket more. I feel bullet failure issues are more common in the smaller calibers like 6.5mm and smaller.

I'm also not a fan of running a tight bore barrel either. A tighter bore...the lands are going to cut into the bullet jacket deeper...your just asking for problems. Been there...and it's happened to me with tight bore 6mm barrels back when I was at Krieger. I did prove it was a bullet problem more than anything else but the tight bore wasn't helping anything. It just made the problem worse.

If you are shooting good quality match type bullets...I don't see the need for a tight bore barrel. You run crappy bullets or bullets that are undersize in diameter....then you might need a tighter spec barrel.

 

[KnowNothing256]

Thanks for the great info, Frank, great to hear real-world first-hand experience.

 

[Fred @ Bison Tactical]

thanks @Frank Green , great information, I appreciate it, I know I have talked your ear off in the past asking these questions, and apologize if you already answered them and I am re-asking. I'm just getting old and forgetful

 

[Frank Green]

thanks @Frank Green , great information, I appreciate it, I know I have talked your ear off in the past asking these questions, and apologize if you already answered them and I am re-asking. I'm just getting old and forgetful

No apologies necessary Fred. Things like this come up all the time. It's a revolving door.

Backing off the twist will not do anything at all for velocity either. Which was one of your questions. So going from a 10 twist barrel to a 12 twist barrel isn't going to really do anything at all.

Also everyone...think of this...... from one barrel to the next a velocity difference up to 100fps....that can just be the individuality of that barrel....still factor in if a different reamer was used etc...

When I said uniformity of the dimensions is one of the key things. One time we had to make a rush order of 6.5CM ammunition test barrels as the ammo maker was having issues with a different brand of test barrels. We got 4 shipped out asap to help keep them running. When I talked to them a week later as I was calling for something else originally...I asked did you get the 6.5CM barrels? They said, "Yep! All have been run in and good to go! All of the barrels pressure wise where running right where they where suppose to be and the velocity was with in 10fps of all of them! Can't make them any better than that. 10fps difference.... that shows uniformity.

 

[TonyTheTiger]

Polygonal rifling will generally provide an increase in velocity over a standard cut rifled barrel,

I've heard that a lot but it wasn't at all the case on the one polygonal barrel I had.

 

[PinesAndProjectiles]

I'll disagree.

We've made polygonal rifle pressure test barrels for a ammunition and bullet maker. They wanted to test different rifling profiles etc...mostly coming at it from a standpoint of accuracy and how it effects long range accuracy on the bullet but they did pressure testing as well.

This was done in this case in 300WM and the polygonal rifling did not produce any higher velocities. So after that test they abandoned pursuing any more testing with polygonal rifling because they actually seen a little drop in velocity and that they didn't want to see at all.

If you look at the total surface area of the bore and you run the same dimensions/tolerances in the bore and groove in a barrel (also regardless of twist) and your using the same chamber reamer spec...you shouldn't see any significant difference in pressures and velocities.

So when a someone tells you this or that makes the barrel faster....you/they better know the details of the barrel. You should also know the bore and groove size to the 4th decimal place. If you don't know the dimensions...now your guessing as to what is going on.

Are the barrels made to the same bore and groove spec dimension wise?

Is the ratio of the groove widths the same?

Is the same chamber reamer spec being used in all of the barrels?

Not to mention ammo being shot thru it. You change any of the above spec...you can effect velocities and pressures as well as bullet diameters will effect things. Here are a couple of examples....

Changing the groove size has a bigger impact in pressures and velocities. I've seen pressure test barrels where the the bore and groove both where -.0005" undersize vs. the min spec. + the tolerance of .0005". That .0005" undersize dimensions raised the pressures 8000 to 10,000psi and drove up the velocities around 150fps. to almost 200fps. Same 6 groove barrels etc...just the one barrels where .0005" undersize on both the bore and groove.

Look at the chamber now...you start changing the throats freebore diameter and or length you can effect pressures and velocities as well. I seen on some 300AAC Blackout test barrels that we made....where the reamer cut the throat only .0001" to maybe .0002" undersize! Depending on which bullet you ran thru the barrels (some rocket scientist spec'd the bullet could be as large as .3090" diameter) well the throat is spec'd at .3090" diameter. So when they ran the bullets that where at .3090" diameter in the barrels (which is size on size with what the reamer is suppose to cut the chamber to and is the start of having problems) and in this case the reamer cut the throat to .3088"/.3089". It bumped pressures up 6k psi and also drove up the velocities. We got the barrels back and got a new reamer and just recut the chambers by hand....all of the barrels pressures and velocities dropped right into spec. That little difference of .0001" or so is all that it took to change things.

So it's more about uniformity and tolerances and dimensions and it's less about the style and number of grooves from everything I've seen.

So one of the things you will hear me say...."the straighter the barrel blank, the more uniform the bore and groove dimensions are over the length of the barrel, the more uniform the twist and the more stress free the barrel blank is....the more forgiving the barrel is going to be."

Later, Frank
Bartlein Barrels

Frank, I just want to say thanks for the explanation.

 

[WindstormSCR]

This feels like sticky material, with so little noise and good authoritative information.

 

[BuildingConceptsllc]

Very interesting stuff.

 

[msgriff]

Is it true that the "R" in 5R stands for "Russian"?

 

[WindstormSCR]

Is it true that the "R" in 5R stands for "Russian"?

It does indeed, the history behind the pattern is interesting, and really has its origins with the russians trying to make quality hammer-forged barrels with a lower scrap rate than traditional rifilng geometry.

"Boots" Obermeyer got his hands on some of the 4-R data courtesy of the US Government while making test barrels for them, and adapted the ideas into the "5R" geometry we know today.

I'm a big fan of the pattern for my rifles simply because it deforms the jacket less, and its a bit easier for barrel makers to create good uniform barrels in.

 

[264win]

Great info above, having the huge expertise and volume of experience from guys like Frank is awesome.
A few years ago some shooting buddies and I were interested in this as well. Trying barrels in 30 cal from 2-9 twist didn’t show any meaningful difference in velocity or accuracy in our small testing samples.
We did find that “ undersized” bullets like JLK would regularly give 40-50 FPS more than
“ standard “ bullets in the same weight and form factor.

 

[Frank Green]

Frank, I just want to say thanks for the explanation.

Your welcome! I wasn't picking on you at all by saying I disagree.

Later, Frank

 

[Frank Green]

Is it true that the "R" in 5R stands for "Russian"?

That is correct!

The "R" does stand for Russian. Not radius!!!!!!!!!!!

Obermeyer took a Russian made/designed barrel and modified it ..... then called it 5R. That's where it comes from.

 

[Frank Green]

It does indeed, the history behind the pattern is interesting, and really has its origins with the russians trying to make quality hammer-forged barrels with a lower scrap rate than traditional rifilng geometry.

"Boots" Obermeyer got his hands on some of the 4-R data courtesy of the US Government while making test barrels for them, and adapted the ideas into the "5R" geometry we know today.

I'm a big fan of the pattern for my rifles simply because it deforms the jacket less, and its a bit easier for barrel makers to create good uniform barrels in.

I agree it does deform/upset the bullet jacket less. This helps fight bullet failure which is a bigger problem in smaller calibers like 6.5mm and under.

I will disagree that it makes it easier for barrel makers to make it easier to make more uniform barrels though. No proof of that anywhere that I've seen. I've also seen plenty of junk/crappy button made and hammer forged made 5R type barrels that shoot poorly.

Every type of manufacturing process for making barrels has it pros and cons. Every process can make a good barrel and a bad barrel.

It comes down to consistency and size uniformity from one barrel to the next etc....and the barrel maker paying attention to what they are doing. You cut corners....you....you get what you pay for. This applies to everyone.

 

[PinesAndProjectiles]

Your welcome! I wasn't picking on you at all by saying I disagree.

Later, Frank

No offense taken. It’s good to get schooled once in a while. Brings a man back to reality.

 

[WindstormSCR]

I agree it does deform/upset the bullet jacket less. This helps fight bullet failure which is a bigger problem in smaller calibers like 6.5mm and under.

I will disagree that it makes it easier for barrel makers to make it easier to make more uniform barrels though. No proof of that anywhere that I've seen. I've also seen plenty of junk/crappy button made and hammer forged made 5R type barrels that shoot poorly.

Every type of manufacturing process for making barrels has it pros and cons. Every process can make a good barrel and a bad barrel.

It comes down to consistency and size uniformity from one barrel to the next etc....and the barrel maker paying attention to what they are doing. You cut corners....you....you get what you pay for. This applies to everyone.

I was thinking more from a metallurgy standpoint, it creates less internal stress points making barrels easier to make consistent.. if as you say, the barrelsmith knows their job!

 

[Frank Green]

I was thinking more from a metallurgy standpoint, it creates less internal stress points making barrels easier to make consistent.. if as you say, the barrelsmith knows their job!

OK following ya now.

I'll stay so no though.

We've even made 4R rifled barrels as well as 2 groove (don't ask for one), 3, 4, 4R, 5, 5R, 6, 7, 7R, 8 groove (not just in 50cal but 22RF and 6mm barrels also) and 10 and 12 groove barrels.

From a cut rifling stand point...no difference in machining. Just might take longer or a little less longer. That's the only real negative with cut rifling....the amount of time to do the rifling. We just made some 58cal, 8 groove barrels. 51" long. First time running that caliber. Each barrel took 8-9 hours on the low end to rifle. Each groove alone was about .010" deep. So it was like rifling five 30cal barrels at one time.

From a button rifling and a hammer forging stand point...I could say possibly.

I still say though regardless you have to pay attention to what your doing. Check out the attached pic. I cannot remember if this was a button made or a hammer forged barrel. The run out in the bore from the o.d. to i.d. was like a .150"! Can't make it up!

Later, Frank

 

[WindstormSCR]

The run out in the bore from the o.d. to i.d. was like a .150"! Can't make it up!

Good grief, though can’t say I’m entirely surprised. Working QC for an aerospace precision machining shop has left me a bit jaded to how bad some “precision” work really is!

And I guess that dovetails into my commentary, from that experience anything that lowers the bar to getting stuff consistent, even if minor is helpful, so even if it doesn’t change the time or care to rifle the barrel itself, the reduced stresses should reduce the chances to get undesirable outcomes in stress relieving and barrel shift when hot.

 

[Frank Green]

I won't type a book on this one....but all of the barrels where the bore went sour from threading the muzzles...all button rifled barrels. (I had nothing to do with the video being made). The last two are our barrels. I know who's the other brands are.

The point he is trying to make with the video is to make the threads as large as you can get away with to help reduce the risk of this happening.

Not saying it can't happen with a cut barrel but I haven't seen it yet. We even had to thread ammunition pressure test barrels for the Gov't and the muzzle threads where 5/8 and it was the last thing we did before cutting and crowning the muzzle and the steel started out at 2.280" o.d. originally.

 

[KnowNothing256]

I won't type a book on this one....but all of the barrels where the bore went sour from threading the muzzles...all button rifled barrels. (I had nothing to do with the video being made). The last two are our barrels. I know who's the other brands are.

The point he is trying to make with the video is to make the threads as large as you can get away with to help reduce the risk of this happening.

Not saying it can't happen with a cut barrel but I haven't seen it yet. We even had to thread ammunition pressure test barrels for the Gov't and the muzzle threads where 5/8 and it was the last thing we did before cutting and crowning the muzzle and the steel started out at 2.280" o.d. originally.

No offense Frank, but a clever and unscrupulous marketer could say the bold sentence even if they were all Bartleins hahaha

Be curious to see the same assessment without changing two variables at once (manufacturer and thread diameter), but your point is taken, and I believe you've made it before elsewhere too.

 

[Fred @ Bison Tactical]

@Frank Green not to take the thread off track too much... But do you think there is any impact with a large OD to the belling on the 5/8x24 barrels? For example if those barrels had started as 1.25 straight contour and were threaded 5/8x24 would there be less belling due to the material left behind the theaded portion?

Finally on the same topic, any down side to threading at a larger diameter say 3/4x24 on a 6mm barrel as long as there is sufficient barrel contour for a shoulder?

Finally a different take my 7 SAUM I shot last year at the Nightforce ELR match is a 1.25 straight contour which is threaded 5/8x24 and I think its the best shooting barrel I have owned...

 

[MDof2]

I fk'n *love* when @Frank Green shows up and drops brain bombs on internet theories.
Love my Barltein barrels.

 

[Frank Green]

@Frank Green not to take the thread off track too much... But do you think there is any impact with a large OD to the belling on the 5/8x24 barrels? For example if those barrels had started as 1.25 straight contour and were threaded 5/8x24 would there be less belling due to the material left behind the theaded portion?

Finally on the same topic, any down side to threading at a larger diameter say 3/4x24 on a 6mm barrel as long as there is sufficient barrel contour for a shoulder?

Finally a different take my 7 SAUM I shot last year at the Nightforce ELR match is a 1.25 straight contour which is threaded 5/8x24 and I think its the best shooting barrel I have owned...

To your first question...I'll say no (depending on how the barrel is made). If you look at my one post where we had to make ammunition pressure test barrels for the Gov't. with 5/8 threaded muzzles for suppressor testing as well. Those barrels started out at 2.280" diameter and we didn't lose a single one and we have to measure the barrel every 1" of barrel length for bore and groove inspection and supply the inspection report to them...as well as when they get the barrels they will go and reinspect them as well. Didn't get a single reject from the bore going sour at all. Order was for 10pcs. of 7.62 Nato testing.

To your 2nd question....as long as you have enough meat on the barrel for the shoulder etc..there is no down side that I can think of other than maybe how it will look depending on what your using.

Awesomeness on your last comment!

Later, Frank