Twist rate calculation

[Ezra]

Can someone provide me a calculation that will allow me to determine optimum twist rate for a particular caliber?   Currently I just use:

(ball diameter) (150) = Twist rate, so for a .32 (using .310 ball) I would get

(.310) (150) = 1:46.5

Speaking of which, what would be generally considered an ideal rate of twist for .32 patched roundball? 

Thanks,


Ez

[LynnC]

I too would be interested in a difinitive answere.  Right now I'm reaming an old .22 barrel up to 24 or 25, gonna make me a pea shooter!  Haven't yet decided the twist rate.  It has been suggested that you should substitute 130 for 150 in the calculation for round ball twist.  Where that comes from I cannot say other than I heard it from one of our Canadian bretheren...........................Lynn

[northmn]

Is their an "optimum" twist?  If you use the diameter X 150 you get 1-75 for a 50 which is pretty slow 130 X 50 which would be 1-65.  Some great groups have been shot with a range of twists in about any caliber.  Also you get into an issue of load type.  The 1-70 became popular in the 50-62 calibers back when because they are supposed to be better with a wider range of loads and permit heavier hunting loads.  Note that most barrel makers use the same twist for a range of calibers.  One of the considerations of twist is to look at the angle of twist.  A 1-70 twist has a rise of .5 in a 50 over 35 inches and a rise of .58 in a 58.  The 58 actually has sharper angle .  If one uses a ratio you get the same rise in a 1-82 inch twist in a 58 and for the 25 a 1-34 twist.  My point is that any formula for "optimum is subject to scrutiny and experience.  Douglas used the 1-66 in about everything and made some very accurate barrels.  By using 1-70 for a 50 as a good twist a .32 would be about 1-44.  If you like a 1-60 in a 50 then it would be about 1-38.  Personally I think small bores really need to be speeded up as they tend to have to be loaded to too high of velocities for small game.

DP
 

[Dphariss]

I think its a waste of time when RBs are the projectiles.
I would probably prefer 48 for calibers to 45 maybe 50.
After that almost anything will work. I would think 66 is just fine for 54 and 72 for most larger calibers 62 etc should be great.
Douglas used 66 for ALL calibers.
I have a .66 with an 80" that shoots better than I can I am sure.

Dan

[jerrywh]

I pretty much agree with Dpharris.   1:48 is good up to .45       For  50 to 54 cal 1:66 or 1:56 is good. Over that 1:72. 

[don getz]

Here we go again, trying to over complicate a simple procedure.   If you start checking, you will find that most, not all,
but most of the old guns had a 1/48" twist.  I'm sure this was arrived at over a long period of time, experimenting with
probably many different twists.  You have to remember, those guys were not interested in shooting magnum loads, they
merely wanted something that gave them accuracy, without wasting a lot of powder.  Even Bob Paris, when he designed
the equipment that we now own, made a rifling machine with a rifling guide that gave his just one twist....1/48".   He did
this in the mid 60's, and during the time he was making barrels, they were considered to be the best swamped barrels
you could get.  Even today, if I had to stick to just one twist, I could easily live with that 1/48"......it will work with any
calibre.............Don

[FL-Flintlock]

Velocity is the key issue because that combined with the twist rate is what gives the resultant RPM on the projectile.  RPM's that are too-high on a round ball are just as detrimental to accuracy as RPM's that are too-low on a conical bullet.

The purpose of applying the twist to round balls is to correct for the minor imperfections in the projectile.  Once you exceed the RPM's required to "correct" for the flaws, the flaws will then become amplified.  Ex:  When tires on your car are spin balanced, they are done so at 45-65 Mph depending on the particular machine.  Normally at speeds below 45 Mph, there isn't enough balance loss to be noticeable because the tire & wheel assembly cannot develop enough mass at the given rotational velocity to be felt through the vehicle suspension.  If you exceed the upper limit by say 15 Mph putting you at 80 Mph, you may start to feel some vibration because now the tire & wheel is developing enough mass from the velocity so that the vibrations are no longer absorbed by the suspension.

The same with a projectile - within a given RPM range, the projectile will be relatively balanced and fly straight.  High quality round balls that aren't deformed upon loading or firing require very little spin to keep them flying straight however, when you put too-much rotational force on the ball, the minor flaws become amplified and the flight path begins to look like a corkscrew because the heavy side will always be pulling on the light side.  This is why with a given load you can get a one ragged hole group at 25yds but at 50yds the group will be opened considerably - but, if you reduce the powder charge thus reducing the MV (muzzle velocity) which reduces the RPM's the 50yd group will tighten up.

Bullet RPM = MV X 720/Twist Rate (in inches)

Using the above formula, a PRB leaving a 1:48 twist bore at 1600fps will be spinning at 24,000rpm.  The same ball leaving the same 1:48 bore at 2,400fps is turning at 36,000rpm.

All things must be taken into consideration too.  If you're shooting a soft lead ball combined with a very tight patch, when you're beating it into the bore the ball is no longer going to be a spherical shape.  The quality of the ball before it's loaded will also make a difference in that voids or contamination within the ball, a large sprue, ball that's not perfectly round, has flat spots or any other mechanical issue will affect accuracy.  

The amount of variables make it impossible to identify a "optimum" twist as the optimum twist will be determined by the variables applied by the user.  Someone running dropped buckshot or whatever balls that fall from the mold will require more RPM's on the ball for a given load than someone who is running very high quality balls.  Same as someone running very light powder charges producing lower MV can get away with a faster twist than someone wanting to run the ball at higher velocity with larger powder charges.  Tight control of the variables are what allows some folks to run smoothbores that are exceptionally accurate with PRB's despite the fact there is no twist at all applied to the ball.

[doug]

     Contrary to those above who feel that one twist fits all (more or less) my experience has been that a fast twist in a 42" barrel is very fussy as to powder charge and powder granulation.  I once owned two barrels by the same maker (Hugh Tonjes) both 54 caliber but one was 1:68 roughly and the other 1:48.  The slow twist barrel did not seem to matter how much or what granulation of powder I put in; group size stayed about the same.  The 1:48 twist had a favourite charge and groups opened up when you departed from that.  Change the granulation and the best powder charge changed dramatically.
     Strangely the short barreled guns that I have owned with fast twists did not seem to show this effect but perhaps because in the case of the originals, I did not load them very heavily.

cheers Doug

[Dphariss]

I had a 50 cal 48" twist Douglas  "Hawken" barrel (yes they made 48 twists at one time).
This barrel would shoot 1" groups at 100, VERY accurate. Probably the best RB rifle I ever owned.
I shot 90 gr of FFFG GOI/Goex 490-495 ball and a ticking patch. Never needed a starter. Just the rod to load.
I do not believe that a very slow twist is actually needed. That is slower than 66 until the bore size is pretty large.  Douglas barrels were all 66 (except as noted above) and all shot very well.
I have had 2 72 twist 54s. On shot well with 100 gr os FFFG and the other simply refused to shoot with less than 120. I would rather have a 48" 50 or a 66" 54 than a 70-72. BUT THEY ALL SHOOT WELL WITH THE PROPER LOAD. So agonizing over twist, at least for most calibers, is not worth it. For 58 and under 56 might be the best. But its impossible to "prove" since a particular 72 twist 54 may outshoot a particular 56 or 66 twist 54. The next 2 may be reversed.
I have a shallow groove (.008) very narrow land .670 barrel with an 80" twist and it tends to fray the patch at the lands with a linen .012" patch thats starts with the rod with a .662 ball. With a cotton ticking patch (about .02-.022)it does not do this and seems to shoot better. But will not load with this patch and a hardened ball and requires a starter with pure lead ticking. Now if it had 100" twist it might not fray (it never "blows" them) the linen patch. Crown is good.
Most patch failures, in my experience anyway, are material deficiencies are the root of this with reasonable fits. I had a couple of yards of ticking some years back that fail maybe 1 shot in 20. Once it was gone the rifle quit eating patches and has not done this in years now.
 
I see no reason to go slower than 66 in the typical deep grooved barrel until past 58 caliber.
Shallow grooves *might* work better with a slower twist but a 5 foot twist is not all that fast and RBs under 58 or so don't have a great deal of inertia to over come.
The last 50 caliber custom barrels I ordered were 48 twist but I have not shot them much yet. However,
These are deep round groove barrels and I already wish they were shallower square. Though I have only shot them enough to get the sights fairly well adjusted.
Trust me the barrel makers know what to use.
It would be interesting to make a 30" twist 54 and see how it worked with powder charges to 1/2 ball weight.

Dan

[Dphariss]

One more thing.
Faster twists are more likely to produce flyers from bad or poorly started balls than a slow twist.
The problem here is that it would take 50 identical barrels from the same maker 25 with one twist say a 50 cal 48 and 25 barrels with a 50 cal 72 to do any kind of a "test" and then it might well be in conclusive.
There are so many factors besides twist involved in accuracy. Just where the barrel is rested when shooting off a rest can be a factor. The long forend on a FS gun is a factor etc etc etc.

Dan

[northmn]

A single twist works pretty well because it is easier to shoot hoter loads in a small caliber, reasonable loads in a medium caliber and lighter loads (ratio of ball weight to powder weight) in a big bore.  Most do.  As Don stated you can make something complicated out of something simple pretty quick.  I will state again that there is no optimum twist.  There are also general rules that state faster twists are better with lighter loads and slower with heavier loads.  About the time you mention this someone like Dan, who shoots a lot and has plenty of experience will state that a 1-48 in a 50 uses 90 grains of 3f and shoots 1" groups.  I do not doubt him at all.  I think they carried the slow twist thing to an extreme.  The big advantage of a slow twist is supposed to be accuracy with less fouling. There are also reasonable cut off points so that one works for a variety of calibers.  My gripe is that for a 32 to have the same angle of twist as a 1-48 50, you would want a 1-32 inch rate.  It is in the smaller bores, I think even the 40,  where we have to load a little too hot to get good results for intended purpose.   It was stated that the original Hawkens had twists of 1-48 and 1-60.  I thought Jaegers were a little faster like 1-32 for their big bores?

DP

[flintriflesmith]

Here we go again, trying to over complicate a simple procedure.   ...  Even Bob Paris, when he designed
the equipment that we now own, made a rifling machine with a rifling guide that gave his just one twist....1/48".   ...  Even today, if I had to stick to just one twist, I could easily live with that 1/48"......it will work with any calibre.............Don

Don,
I agree completely. Only folks loading up bench type guns to get wind bucking velocity need slower twists. (Or claim they do.)

I got one of Bob Paris's first .60 caliber barrels and it was a 1-48 twist. With a .585 ball and army fatigue cloth patching, pushed by 110 grains of DuPont FFg, it would consistently cut one ragged hole at 50 yards. Many experts have told me that was too lose a load with too much powder. Glad the rifle couldn't hear the experts!

Gary

[Ezra]

This is all very interesting.  So it appears there is no "optimum" and it also seems (at least to me) that a squirrel rifle that is shot at 40 yards or less, may be just as well served as a smoothbore.  By smoothbore I mean a .25, .30. .32, .36 in smoothbore.  Hmmmm.


Ezra

[Dphariss]

This is all very interesting.  So it appears there is no "optimum" and it also seems (at least to me) that a squirrel rifle that is shot at 40 yards or less, may be just as well served as a smoothbore.  By smoothbore I mean a .25, .30. .32, .36 in smoothbore.  Hmmmm.


Ezra

If this were true rifles would not have been invented in the first place. Then we must ask why people would pay to have their squirrel guns rifled if a SB would work. This is pure silliness.
I sure that there is not a smoothbore in existence that will hit a squirrel every shot a 40 yards, I doubt many will do it at 25.  Even in the body, much less the head.

Dan

[Ezra]

Perhaps.  Still, a very smallbore smoothbore would be interesting.  Just to see what it would do.


Ez

[Dphariss]

Perhaps.  Still, a very smallbore smoothbore would be interesting.  Just to see what it would do.


Ez

You seem to think that since there is not set twist for a RB barrel of a given caliber that there is an accuracy problem. There is not. The discussion of twists is a tempest in a teapot. The RB shoots very well with a wide variety of twists. I suspect that if all other factors are equal that a 45-50 caliber rifle barrel with any twist from 40-72" will shoot as well as a barrel with some carefully calculated twist will once the best load it found. I doubt it can be proven that a 48" twist is any more load sensitive than a 66. John Baird and TK Dawson tested an original rifle with a 48" twist 54 barrel and found that a composite group of four 5 shot groups at 50 yards with 70, 90, 110 and 130 gr of FFG resulted in a 20 shot group measuring 2"W x 2 3/4" H and 130 gr produced the best groups.
Just because people here have different opinions concerning rifling twists does not mean that there is an accuracy problem. What is shows is that the twists in general use by various barrel makers ALL WORK WELL.
The phobia over twists comes from the elongated bullet world of ML slug guns and the brass suppository guns. Here even a couple of inches difference in a twist can make a lot of difference to a given bullet length or shape. Buts its comparing apples and oranges. The round ball simply is not that fussy.

Dan

[northmn]

I think  that maybe extremes may make a difference.  In the 25 cal thread, they mentioned that a little 25 uses about 20 grs of powder behind a 24 gr ball for accuracy.  Most have found the little 32 to need a heavy charge such as 20 to 25 grains.  Daryl has also suggested heavier charges in a 40.  That is because the angle of the twist (which is what twist rate really is) is so low on a small bore.  A 25 with a 1-48 twist is equivalent in angle to a 50 with a 1-96.  RB's are not all that fussy I agree, but I really would not want a RB twist at 1-96 for a 50.  At a point you almost get a straight rifled barrel.  Some folks, on the bench, used to shoot some very heavy charges out a 54's, like 150 grains and wanted a little slower twist which may again contribute to less fouling also.   As to accuracy differences between normal twists barrel makers use, it likely does not mean much as we match the load to the rifle anyway and also bigger bores tend to be shot with a smaller ball to powder ratio.  I used to shoot Numrich barrels which had 1-56 inch and held my own with them.   

DP

[Dphariss]

Hi Northmn
One must also remember that the smaller bores are far less efficient than the larger  bores in turning a grain of BP into FPS on the ball.
My 16 bore rifle shooting a 1 ounce ball will make 1650 with 150 gr of FFG. This is about 35% of the ball weight.
35% of a 310 ball is about 16 grains of powder. This is not going to make 1650 fps. Its going to need 2025 grains at least, about 50% or more of the ball weight.
So the smaller bores need more powder simply because the lighter ball has less inertia and it does not burn the powder as well.
Yes, a 32 should shoot better with a faster twist than that typically used for a 54. But I see no reason to go faster than 48".

Dan

[northmn]

I suspect the 1-48 is an economy thing for barrel makers.  The reason I suggest a faster twist is the use of the small bores.  A 32 with the 20-25 grains charges are about like using a 22 mag HP which isn't used much on small game.  I am talking about squirrels and rabbits, etc.  Head shots are best, but sometimes one pulls a little and they really blow up a critter.  Unneeded power.  I am familiar with the powder differences as well.  3-f in a small bore is similar to 2f in a larger bore.  As to the need of changeing.  I do not feel so strongly about it that I am going to order a special barrel with a special twist to do so.  It would be fun to experiment with but I have other experiments I would rather spend my money on.  I have taken off a lot of heads with a 32 and now use a 40.  In a way that is a challenge of using a ML.  Actually my 40 with a 45ACP case of 3f (about 30 grains) does less damage to the body of a squirrel than a 32 with a lighter charge. 

DP

[doug]

      the 100 - 120 grain loads in a .50 cal would be consider a hot hunting load around here rather than a target load.  On our trails, we shoot more in the range of 50 - 60 grains and I think it is at those velocities that accuracy becomes fussy with load when shooting a "fast twist" barrel

cheers Doug

[Ezra]

"You seem to think that since there is not set twist for a RB barrel of a given caliber that there is an accuracy problem."


No, not really.  The concept of this whole thread was to elicit comments from those more educated in twist rate than I.  Which is pretty much everyone.  I have drawn no conclusions regarding twist rate as it relates to caliber, velocity or powder charge when it comes to shooting roundball.  Seems like black magic to some degree to me.


Ez