Formula for optimum rate of twist?

[David R. Pennington]

Has anyone ever presented a mathmatical formula for determining optimum twist rate for prb barrels? I have heard a couple raw theories, but seems like a good math problem. Typically smaller calibers shoot better with faster twist etc.,. Should be some mathematical equation to relate ball weight or size to rate of twist. Would you need to take barrel length into the equation? I heard one maker touted 3/4 turn in your barrel length?

[smylee grouch]

You might want to consider the velocity you will be pushing the ball with too

[L. Akers]

Google Greenhill Formula.

[Pukka Bundook]

L Akers has it . (Above)

[RichG]

http://kwk.us/twist.html Here's a link to a formula i found on line. the greenhill formula isn't perfect for roundballs and velocity is important. with the larger calibers you might need to balance optimum twist with just how much recoil you're willing to put up with.

[Daryl]

Note that the Greenhill Formula was low velocity bullet oriented.

The twist # that is the suggested from the formula is the or very close to the SLOWEST twist that will work - it is not the optimum.

[hanshi]

The formula I had drilled into me was, "not too slow and not too fast".

[Frank]

I remember reading some articles in The Buckskin Report and also reading in John Baird's books, 15 Years in the Hawken Lode and Hawken Rifles, The Mountain Man's Choice. John had shot some original Hawken Rifles and had some fantastic groups at 100 yards ( 2 inches or less). Those guns were all 1 in 48 twist, and were 50 and 54 caliber rifles. I have always favored barrels with a 1 in 48 twist, but those are almost impossible to find now.

Maybe Rice will cut those on a special order basis, but I don't know.

[smylee grouch]

There are a lot of custom barrel makers who will cut what ever rate you want. Rayl,Burton,Carpenter to name a few.

[Frank]

Bobby Hoyt as well. Just thinking Rice because their profiles are common with a lot of stock makers. Don’t care for precarves , but want the barrel fully inletted on a swamped barrel. Of course a straight barrel is always an option. Just been spoiled by swamped barrels as that is all I have used the past 20 years.

[ScottH]

Rice lists 1 in 48 twist for .32 and .40 caliber barrels. So they can definitely rifle a barrel with that twist. I do believe that you will pay a bit extra for a non standard twist rate for larger bores.

[Elnathan]

There are a lot of custom barrel makers who will cut what ever rate you want. Rayl,Burton,Carpenter to name a few.

Who is Carpenter? Haven't heard that name before.

[L. Akers]

I've been rifling my own barrels since the 70's using Greenhill's formula ( which was actually developed for artillery).  For a round ball, the formula reduces to 1 turn in 150 calibers; ie for a .32 the formula is .32" X 150 = 48"--Rice's twist for their .32 as stated above.  A round ball does not need a lot of rotational velocity to stabilize.  The first barrel I rifled was a .43 caliber and had a 1 turn in 96" twist (I hadn't heard of Greenhill at the time) and is supurbly accurate.  It has been stated that velocity needs to be considered also ( true) and that is directly related to powder charge.  The faster the twist in a given caliber the more particular will be the "optimum" powder charge ( velocity).  I have found a slower twist is more "forgiving" of powder charge ( velocity) without sacrificing accuracy.  The slower twist seems to give a wider "optimum" window. 

To answer your barrel length question, David, the barrel, regardless of twist rate, has to be long enough to burn your powder charge, no matter what amount that is, before the bullet exits the muzzle.

[yulzari]

and then of course there is Forsyth Rifling which applies to large balls allowing large charges.

[smylee grouch]

Jim Carpenter, St. Maries, Idaho. One of the better ones. 208-245-3693

[L. Akers]

and then of course there is Forsyth Rifling which applies to large balls allowing large charges.

Although Forsyth rifling entails more than just a slow twist, I found it interesting that the twists he mentions in his book are very close to what Greenhill's formula gives for the calibers mentioned.

[Bob Roller]

I remember reading some articles in The Buckskin Report and also reading in John Baird's books, 15 Years in the Hawken Lode and Hawken Rifles, The Mountain Man's Choice. John had shot some original Hawken Rifles and had some fantastic groups at 100 yards ( 2 inches or less). Those guns were all 1 in 48 twist, and were 50 and 54 caliber rifles. I have always favored barrels with a 1 in 48 twist, but those are almost impossible to find now.

Maybe Rice will cut those on a special order basis, but I don't know.

On Thanksgiving Day of 1967 I finished and tested the only "Hawken"I ever made.
It was a Bill Large barrel,34"long and 1-1/8" ATF and .54 caliber with solid bolster breech
and a turn in 57".I used a soft lead .535 ball over 100 grains of DuPont 3fg and it would
do under 2"groups easily**. It was an 8 groove barrel.I was 31 when I did this good shooting and now
I doubt if I could do anywhere close to that with the sights I had then that were copied from an
original J&S.Big differences between 31 and 82 and no practice shooting for years. Bill made a lot
of 1 in 57 twist barrels and some even with Pope style rifling that were much in demand.
The late Don Brown told me of 58 caliber barrels made as an experiment for his Alex Henry copies
with rifling that looked like the splines on an input shaft of a car transmission and using 120 grains
of 3fg got over 2000 FPS and with good accuracy.

Bob Roller
**100 measured yards on a gun club range.

[Darrin McDonal]

In all honesty, round balls (aka bullets) are very forgiving when it comes to the rate of twist but the powder charge to push the bullet for the optimum accuracy is the key. It's not about how much powder you could use or the rate of twist it's the perfect powder charge to match that bullet size and rate of twist.
The best way to do it is to take the weight of your round ball divide that in half by grains of powder and start shooting on target. Slowly reduce your grains continuing to shoot a group on target until you found the optimum powder charge. Will end up using less powder than starting somewhere in the middle and then either working up or down.
Darrin

[Daryl]

and then of course there is Forsyth Rifling which applies to large balls allowing large charges.

Forsyth noted 1/4 turn in the length of the barrel - that was "his" formula. He also referred to SxS rifles, with
24" to possibly 26" bls, thus 96" to 104" twists. Most singles (16 to 14bore) of that period had 30" to 31" bls.
thus would have 120" to 124" twists. Forsyth mentioned twists as slow as 12 feet, able to give adequate accuracy
to 150yards. However, if wanting to shoot further, to 200 or 250yards, a twist rate of 1 turn in 8'8" would work, if
made correctly. This is, of course, his famous 104" twist rate.

[smylee grouch]

I have for some time wanted to make a 66 cal (.660 bore shooting a .645-.650 ball ) and often wondered if a 1-85 twist would work with a 4 dram load. Hoping to keep recoil down some with that light of charge.

[Daryl]

I have for some time wanted to make a 66 cal (.660 bore shooting a .645-.650 ball ) and often wondered if a 1-85 twist would work with a 4 dram load. Hoping to keep recoil down some with that light of charge.

DPhar could answer that, however he is on the road at this time.  His English gun has a 16 bore Dave Rayle barrel. I think it has an 85" twist, shallow grooves (I think) and he uses just over 5 drams for best accuracy, IIRC.





Also - if I remember correctly, Dave makes his 16 bore barrels (any twist you want) with a .665" bore, so you can use a full 16 bore ball, ie: .662" Lyman mould.

[Bob Roller]

I have for some time wanted to make a 66 cal (.660 bore shooting a .645-.650 ball ) and often wondered if a 1-85 twist would work with a 4 dram load. Hoping to keep recoil down some with that light of charge.

Smylee,
I would think that load under a 16 bore round ball would as the late
Tom Dawson once said "Turn a critter Arse up in the berry patch"
I had a sxs Greener 16 bore double rifle that was potent enough with 90
grains of 3fg.

Bob Roller

[smylee grouch]

You might be right Bob and if I do build such a gun I would hope your right too. The 1-85 twist was what I thought might work for a 4 dram load, I know a lot of those English bigger game guns had the 1-104 or 1-144 twists but they to my way of thinking would need more powder to reach their best acuracy so I would want to go with a smaller load to group good and still have power enough for what ever I at my age will do or hunt with it. A .645 ball should weigh 400+ grains and one person sugested startin g loads of half the ball weight in powder, ha not this old timer. Four drams (110 grns.) sounds like a good compromise for this shooter.

[WadePatton]

The formula I had drilled into me was, "not too slow and not too fast".

There are a lot of custom barrel makers who will cut what ever rate you want. Rayl,Burton,Carpenter to name a few.

My current formula (although I used to ponder these things at length) is now "BBL maker knows best". 

I prefer to let him decide-according to what the gun is to be used for. It's their business to know these things, and mine to do other stuff. 

[Hungry Horse]

Without the caliber, and what the gun will be used for, answering your question is kind of like the old joke question” what’s the difference between a duck”.

 It all depends on those two things. Big calibers can shoot well with a fast twist at close range with light charges. And, small calibers can shoot well at close and moderate range with slow twists and heavier charges. But, you have to decide what use the gun will be put to. The 1 in 48” twist was considered a universal twist because it did pretty well out to a hundred yards with most calibers.

  Hungry Horse