Understanding the different rate of barrel twists and most compatible projectiles

[drpatton]

I see what you wrote on #16. With your comment, I will stand corrected. I understand you have a friend who was able to accomplish this. There must be something which allowed him to do it. One of the variables must have been changed to allow it. (deeper grooves?)

There are several rifles with a 1:32 twist which maintain the deep grooves. I own a few. The deeper grooves could allow for the PRBs to work.

Your last paragraph, basically states what I said. Given a fast twist rate, shallow grooves and a high charge, the rifle tends to become a smooth bore.

I have never tried using RBs in a 1:28 twist barrel, but there are several videos on YouTube of people who tried, and all have failed. Your friend should make a video to prove to those who failed, it can be done.

 

[CatamountRob]

The small caliber rifles are all over the place as far as twist rates go. The Traditions Crocket is 1:48 while the T/C Cherokee (and probably the Seneca?) are 1:30. Probably some even slower than a 48 that I’m not familiar with.

 

[ETipp]

The small caliber rifles are all over the place as far as twist rates go. The Traditions Crocket is 1:48 while the T/C Cherokee (and probably the Seneca?) are 1:30. Probably some even slower than a 48 that I’m not familiar with.

Yet the KY long rifles are largely 1:60" to 1:66" twist. And they are meant to be used for RB shooting.

 

[Ka'imiloa]

I see what you wrote on #16. With your comment, I will stand corrected. I understand you have a friend who was able to accomplish this. There must be something which allowed him to do it. One of the variables must have been changed to allow it. (deeper grooves?)

There are several rifles with a 1:32 twist which maintain the deep grooves. I own a few. The deeper grooves could allow for the PRBs to work.

Your last paragraph, basically states what I said. Given a fast twist rate, shallow grooves and a high charge, the rifle tends to become a smooth bore.

I have never tried using RBs in a 1:28 twist barrel, but there are several videos on YouTube of people who tried, and all have failed. Your friend should make a video to prove to those who failed, it can be done.

Thanks for your thoughtful reply.
Unfortunately, the man who owned the inline rifle passed away some time ago, and the other has moved to the big city and given up his shooting and hunting due to age. I certainly would not have believed their results with round ball had it not been for their astuteness and long experience with MLs and RB shooting. And I sure do remember their test report to me!!
That second friend, well known in ML circles here in the SW, was fond of pocket drill as the best patch for such a trial, and indeed for other ML shooting. He got me to using it. It is uniform, thinner than pillow ticking (which measures around .015" compressed), and also smoother in surface -- so it goes down bore a bit easier even with a tight-fitting patch-ball combination.
BTW, if you need a really thin patch, use a quality silk handkerchief or scarf (be sure to check label). I needed that for a belted-ball load, and managed to find an old reference that silk was the patching on belted balls (and perhaps "belted" slugs too). My silk patch measured .003"compressed.
Use a felt wad beneath that thin patch of course, and when using such a wad under a RB, make it an oversized one, such as 54 or even 58 cal. size in a 50 cal. bore, seeking to maintain a bore-seal.
Aloha, Ka'imiloa

 

[Patched]

Soon after getting back into muzzleloading in 2000 i began firing round balls in inline and conventional muzzleloaders. i've fired round balls very accurately from rifles with twist rates as fast as 1:32. My old CVA Stag Horn with it's deeper rifling is very accurate with 70 grains of powder and a patched round ball. The .54 Fire Hawk with 1:38 twist rate is very accurate with 80-90 grains of Black MZ powder and a tight fitting patch: At 100 grains accuracy begins to decline.

My .54 caliber Navy Arms Hawken barrel done by Mr. Hoyt has rounded groove rifling and 1:66 twist rate. It makes one 5 shot hole at 50 yards.

The compromise thing with the TC guns is the shallow rifling accompdation for conical bullets. Patched round balls can be accurately fired from those guns, but a tight fitting patch is necessary for use with larger powder loads.

There is this thing about very slow twist rates: Balls driven by different charges of powder land in the same group.

Here's Doc White:

http://whitemuzzleloading.com/round-balls-in-fast-twist-rifles/

Doc White is the guy with the heavy weight credibility…but I played with these guns for 50 years also.
Doc White is right on point with my experiences….a PRB ”skips” the rifling with a faster twist and shallow grooves…way less powder works.

1:72 twist with 12thousandths deep round bottom grooves is ideal for thicker patches and maximum powder charges…so maximum that you need a felt wad atop to protect the patch to retain your accuracy…real linen in the correct thickness is the perfect patch material period.

1:48 twist deep grooves (like Hawken originals) work somewhat with a heavier powder charge accuracy wise but never as well as the 1:66 to 1:75 twists
Elongated conical bullets dictate their twist requirements by their length and the shallow grooves work to their advantage.

My 6 cents worth from many projectile launches from many manner of launch platforms….

 

[Ka'imiloa]

Well said. Hope to hear more of your "6 cents worth" comments.
The Doc White link you gave us is priceless, and I hope a whale of a lot of folks on this Forum read it. He's simply a ML guru.
He relates using 3F BP in his .45 RB loading in shallow-rifled modern barrel. The peak pressure from a given volume of 3F is higher than with 2F or 1.5F. The brand of powder counts too, such that Swiss BP, or the original Triple 7, would give the same velocity as Goex or Pyrodex R with approx. 15% less powder by volume. I believe this has been tamed down somewhat now. Olde Eynesford also came out as a stronger powder, and I don't know how that situation is now.
If Doc White had used 2F or 1.5 F powder for his inline/roundball test, which burns a bit slower and results in lower peak pressure, he might have been able to use higher loads of powder and still maintain accuracy in his shallow-rifled inline barrel, by extending the time of the powder burn in the barrel.
This would possibly allow the RB in its patch to accelerate into its high rate of spin without "stripping the rifling" and thus failing to have enough spin-rate at the muzzle to maintain accuracy. Esoteric stuff, but all this matters in internal ballistics, especially with bullets or balls that have to be loaded from the muzzle.
One thing of note in this thread: the bigger the ball, the more mass it has, and thus the more self-stabilization. So the bigger the bore, the less twist rate you need to stabilize a round ball. Hence, traditional 1:66 twist for a .45 or .50 barrel. But as you go larger in bore size, 1:72 twist works fine for 54 cal., and even larger bores get by with even slower twist rates for a round ball.
The 1:48 twist of a T/C ML is a compromise twist rate so as to shoot both RBs and bullets, as you probably know.
Aloha, Ka'imiloa

 

[drpatton]

Yet the KY long rifles are largely 1:60" to 1:66" twist. And they are meant to be used for RB shooting.

The slower twist rates (1:60 and slower) are meant as round ball shooters. I read many, many years ago, that the perfect twist for a round ball in a .54 is 1:72. Now where they got that, I don't know.

 

[Hanshi]

Twists of 1-48" were common in the old days. The Hawken brothers used that twist in their "Hawken" rifles. Rule of thumb #1. The smaller the ball the faster the twist. #2. The larger the ball the better they work in slower twists.

Within reason the twist of the grooves is not nearly as important as the DEPTH of said grooves. Shallow groove barrels, regardless of twist, can let a prb "strip" the grooves. Deeper grooves allow thicker patches and tighter loads to be used and the grooves get a good "bite" on the prb. Calibers such as .32, .36 and .40 do well with 1-48" and larger ball, .45, 50, etc, can do just a well with that twist; but the grooves need to be at least .006" or deeper. Most of my rifles have grooves of .010" all the way up to .016". A much used .45 of mine and a .50 have twists of around 1-56"; the .54 is 1-66" with a depth of .006".

So don't get caught up in the twist confusion and shoot what your rifle will like including prb.

 

[Patched]

The slower twist rates (1:60 and slower) are meant as round ball shooters. I read many, many years ago, that the perfect twist for a round ball in a .54 is 1:72. Now where they got that, I don't know.

Actually someone got that twist rate by trial and error. I have 1:75 54 cal barrels. As many here know, James Forsythe wrote a book around 1850 on “The Sporting Rifle” laying out arguments for twist rates and projectiles based on testing and first hand experiences hunting.…apparently people were experimenting constantly… mostly military… common folk stuck with what was known to work and 1:48 had been working for a while.

1:72-75 twist takes advantage of maximum charges of the limited power of black powder in 54 and 58 cal and still retains accuracy.

 

[drpatton]

Twists of 1-48" were common in the old days. The Hawken brothers used that twist in their "Hawken" rifles. Rule of thumb #1. The smaller the ball the faster the twist. #2. The larger the ball the better they work in slower twists.

Within reason the twist of the grooves is not nearly as important as the DEPTH of said grooves. Shallow groove barrels, regardless of twist, can let a prb "strip" the grooves. Deeper grooves allow thicker patches and tighter loads to be used and the grooves get a good "bite" on the prb. Calibers such as .32, .36 and .40 do well with 1-48" and larger ball, .45, 50, etc, can do just a well with that twist; but the grooves need to be at least .006" or deeper. Most of my rifles have grooves of .010" all the way up to .016". A much used .45 of mine and a .50 have twists of around 1-56"; the .54 is 1-66" with a depth of .006".

So don't get caught up in the twist confusion and shoot what your rifle will like including prb.

I don't agree with you in part. Yes, the Hawken Bros. did make their rifle with a 1:48 twist. To accomplish this, they had to make the groove depth deeper. This allowed the lead PRB to stay engaged in the rifling.

From Wikipedeia: "In 1879, George Greenhill, a professor of mathematics at the Royal Military Academy (RMA) at Woolwich, London, UK developed a rule of thumb for calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet's length, needing no allowances for weight or nose shape. The eponymous Greenhill Formula, still used today, is:

where:

• C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
• D = bullet's diameter in inches
• L = bullet's length in inches
• SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

For pure lead bullets, the second part of the equation = the Square root of 1 and factors itself out of the equation.

So, for a .530 caliber round ball, the length is equal to its diameter.
150 x .530^2/.530 = 42.135/.530 = 79.5

To calculate for a .540 then it's 150 x .540^2/.530 150 x .2916/.540 = 43.74 / .540 = 81

The best twist rate for a .530 patched round ball is 1:79.5"
The best twist rate for a patched .540 RB is 1:81"

The problem occurs when a twist which is too fast for the lead to stay engaged in the rifling; then groove depth must be increased. That said, the formula shown above indicates the extra rotation of the bullet doesn't assist in accuracy. It just places more spin (angular momentum) onto the round ball, which robs the RB of forward velocity or forward energy. The energy is lost in rotation and additional heat from the extra friction in the barrel.

 

[Patched]

I don't agree with you in part. yes, the Hawken Bros. did make their rifle with a 1:48 twist. To accomplish this, they had to make the groove depth deeper. This allowed the lead PRB to stay engaged in the rifling.

From Wikipedeia: "In 1879, George Greenhill, a professor of mathematics at the Royal Military Academy (RMA) at Woolwich, London, UK developed a rule of thumb for calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet's length, needing no allowances for weight or nose shape. The eponymous Greenhill Formula, still used today, is:
View attachment 29363
where:

• C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
• D = bullet's diameter in inches
• L = bullet's length in inches
• SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

For pure lead bullets, the second part of the equation = the Square root of 1 and factors itself out of the equation.

So, for a .530 caliber round ball, the length is equal to its diameter.
150 x .530^2/.530 = 42.135/.530 = 79.5

To calculate for a .540 then it's 150 x .540^2/.530 150 x .2916/.540 = 43.74 / .540 = 81

The best twist rate for a .530 patched round ball is 1:79.5"
The best twist rate for a patched .540 RB is 1:81"

The problem occurs when a twist which is too fast for the lead to stay engaged in the rifling, groove depth must be increased. That said, the formula shown above indicates the extra rotation of the bullet doesn't assist in accuracy. It just places more spin (angular momentum) onto the round ball, which robs the RB of forward velocity or energy.

I was hoping someone would throw down the Greenhill formula (45+ years after Forsyth)

Of course a round ball is the shortest and lightest lead projectile to fit a given bore size and should therefore require the least amount of spin to maintain a stable flight to the target…in 54 cal 1:75 vs 1:79.5 is erring on the beneficial side of the stabilization formula.
Losing velocity/energy as quickly as a RB does, it keeps itself a 125 yd prospect…perfectly suited for the relative low energy of black powder.

Elongated conical bullets come of age when the smokeless powders can drive them to and above round ball speeds.
…another 4 cents worth of opinion.

 

[drpatton]

I was hoping someone would throw down the Greenhill formula (45+ years after Forsyth)

Of course a round ball is the shortest and lightest lead projectile to fit a given bore size and should therefore require the least amount of spin to maintain a stable flight to the target…in 54 cal 1:75 vs 1:79.5 is erring on the beneficial side of the stabilization formula.
Losing velocity/energy as quickly as a RB does, it keeps itself a 125 yd prospect…perfectly suited for the relative low energy of black powder.

Elongated conical bullets come of age when the smokeless powders can drive them to and above round ball speeds.
…another 4 cents worth of opinion.

I'm surprised it wasn't brought up before this.
The Greenhill formula works on copper jacketed bullets and even alloys. To use it, one must know the Specific Gravity or Density of the alloy or jacketed bullet. Then you divide the actual SG by the SG of lead and apply its square root. Also, with velocities above 2,800 f/s, the value of "C" becomes 180.

What Greenhill proves is even a twist of 1:65 is wasted rotation and energy in a .50 with a ball of .495 (1:74.25")

 

[drpatton]

I was hoping someone would throw down the Greenhill formula (45+ years after Forsyth)

Of course a round ball is the shortest and lightest lead projectile to fit a given bore size and should therefore require the least amount of spin to maintain a stable flight to the target…in 54 cal 1:75 vs 1:79.5 is erring on the beneficial side of the stabilization formula.
Losing velocity/energy as quickly as a RB does, it keeps itself a 125 yd prospect…perfectly suited for the relative low energy of black powder.

Elongated conical bullets come of age when the smokeless powders can drive them to and above round ball speeds.
…another 4 cents worth of opinion.

I wonder what would happen if a barrel company came out with a .54 groove diameter barrel with a 1:75 twist rate. My guess is, no one would buy it, even though, that would probably be the optimum twist rate for PRB at that caliber.

 

[Greg_E]

What diameter should be used for something like the Lee REAL bullets? Do we take it at nominal bore diameter of .450, .500, etc.?

Just an off hand guess, but I'm guessing the 200grain 45 version will come out at about 1:60 or 1:66. I'll have to measure the mold later since I haven't cast these yet. I'll measure both 50 cal since I have both cast.

 

[Greg_E]

I wonder what would happen if a barrel company came out with a .54 groove diameter barrel with a 1:75 twist rate. My guess is, no one would buy it, even though, that would probably be the optimum twist rate for PRB at that caliber.

You can get smoothbore which seems to be pretty accurate with no spin.

 

[drpatton]

What diameter should be used for something like the Lee REAL bullets? Do we take it at nominal bore diameter of .450, .500, etc.?

Just an off hand guess, but I'm guessing the 200grain 45 version will come out at about 1:60 or 1:66. I'll have to measure the mold later since I haven't cast these yet. I'll measure both 50 cal since I have both cast.

I need to know the Length of the REAL bullet. Also, the REAL bullets don't have a lot of bearing surface to engage the rifling.

Let me know the actual diameter or caliber of the bullet and its Length and I will let you know the given twist rate for it.

 

[drpatton]

A .442 conical with a length of 2" will need 1:14.5 twist rate.

A .432 conical with a length of 1.75" will require a 1:16" twist

What I don't know is with these quick twist rates, will the lead stay engaged in the rifling or would an alloy be required? There is a point where the density of lead is too soft to stay in fast twist barrels. I don't know what that point is.

If there is a Mathematician in here, please calculate it. Please use a .50 caliber, a groove depth of .005, 6 lands, 6 grooves evenly spaced and a temp of 75*F (24*C). Use the SG of lead as 10.9 to stay within the formula. At what velocity does the lead start to strip through the rifling in a 1:28 barrel? Or use 1,250 f/s as a standard velocity to find the twist rate of failure.
Did I cover all of the variables?

 

[Greg_E]

I just used nominal diameter which is probably pretty close once you take the drive bands into consideration.

The 45 200 grain is approximately .570 long and gives me around 1:53 so it may not work great in my 1:60 barrel.
The 50 250gr is .582 which comes out to 1:64.
The 50 320gr is .720 which comes out to 1:52 which will probably work well in my 1:48 barrel and has already shown to group decently for me.

I did recalculate both 50 using the actual body of the bullet at a measured .462 and the results were different enough to make me think a bit. Got a 1:52 for the 250 and 1:44 for the 320. Again, I have a feeling that the effective diameter is going to be close to .495 to .505 for my somewhat deep groove barrel (.495 and .520 as roughly measured at the muzzle). The 320 is probably close enough, but I may find the 250 to be more accurate when I try them.

And all this math is fun, but the rpm is going to vary wildly with the velocity, so I don't think anyone should use these for more than a rough guide for selecting your starting points.

 

[Greg_E]

I just went to check on these REAL molds to see my options in 45, and noticed a link to the Greenhill calculation in the description of the mold.

https://support.leeprecision.net/en/knowledgebase/article/greenhill-formula
I wonder how that 45 mold would do if I milled the first band off? It would certainly relax the twist needed.

(edit) looks like I would need to push that 45 bullet all the way out to the faster range of 2000+ fps to make it work, I might need to sacrifice the round ball half of the mold and mill it down to remove one of the drive bands, or have Accurate cut me a mold. Cheaper to mill off the one I have now for testing. I'll have to cast and cut some of the results to a shorter length and see if I see anything.

 

[drpatton]

I just used nominal diameter which is probably pretty close once you take the drive bands into consideration.

The 45 200 grain is approximately .570 long and gives me around 1:53 so it may not work great in my 1:60 barrel.
The 50 250gr is .582 which comes out to 1:64.
The 50 320gr is .720 which comes out to 1:52 which will probably work well in my 1:48 barrel and has already shown to group decently for me.

I did recalculate both 50 using the actual body of the bullet at a measured .462 and the results were different enough to make me think a bit. Got a 1:52 for the 250 and 1:44 for the 320. Again, I have a feeling that the effective diameter is going to be close to .495 to .505 for my somewhat deep groove barrel (.495 and .520 as roughly measured at the muzzle). The 320 is probably close enough, but I may find the 250 to be more accurate when I try them.

And all this math is fun, but the rpm is going to vary wildly with the velocity, so I don't think anyone should use these for more than a rough guide for selecting your starting points.

The bullets should expand in the barrel to the groove width, when fired. So, I would use the groove diameter for calculations.

As for a rough estimate? I agree and so did Dr Greenhill. It was a quick way of getting to the correct twist. From my understanding, it is pretty accurate. That said, just because your "45 200 grain is approximately .570 long and gives me around 1:53 so it may not work great in my 1:60 barrel." It will do fine. A 1:70 may be an issue. But I believe you can shoot it without any problems.

There are people who have shot a stock T/C .54 renegade at 500 yds with a 1:48 twist. The bullet probably should have had a twist around 1:36 or quicker, but he hit the gong.

My best educated guess is, the smaller the caliber, the more important the twist rate. I would also use a slightly quicker twist than calculated with Greenhill's Formula for smaller caliber rifles.