Mike R.

Can someone explain to me the rate of twist and how it relates to the barrel length?

For example: This was printed in an ad for a Lyman Great Plains rifle. "This classic muzzleloader offers such high quality features as a 32" barrel with 1 in 60" twist for patched ball and hunting loads."

I think I understand the twist rate, but wouldn't a 1 in 48" provide better accuracy? Or is that rate only available in shorter barrel lengths?

arcticap

Although it seems that slow twist barrels are usually longer and medium twist barrels are usually shorter, the rate of twist is quite independent of barrel length.
A couple of examples are the Cabela's/Pedersoli Blue Ridge rifles which have 39 inch barrels and a 1 in 48" twist, and the Traditions Crockett rifle which is a 1 in 48" 32 caliber with a 32 inch barrel.

http://tinyurl.com/mmtvto


There have been a few short slow twist gun barrels in the past but not very many.
Accuracy is based on stabilizing a projectile by spinning it with the correct amount of velocity creating revolutions per minute (RPM's). The optimum amount of RPM's required is based on the length of the projectile and the Greenhill formula. The longer the projectile, the more spin that's needed to stabilize it.
Balls are shorter and require less spin, depending on caliber.
Conical bullets are longer and require more spin, how much depends on their length (which is also related to their weight).

Adding more velocity increases RPM's until the sweet spot which produces better accuracy is found.

To reduce RPM's, load less powder.
To increase RPM's add more powder.

So accuracy depends on the length of the projectile being fired and how much velocity and RPM's is imparted to it. Conicals usually do require a faster twist unless it's a very short concial like a Buffalo Ballet which can be driven at high velocity from a slow twist barrel like the Great Plains.
If one wants to shoot a patched round ball at hunting velocity, then a slow twist is usually better but not always. It also depends on the quality of the rifling itself.
If one wants to shoot a longer conical, then a relatively faster twist producing more RPM's is required.

To reiterate, fine tuning RPM's is controlled by the amount of powder.
The choice of twist rate should be influenced by which projectile is intended to be fired for the primary purpose of the rifle.

driftrider

Twist rate and barrel length are completely independent of each other. A 1:48" twist barrel can be 36" long, or 6" long, but if the muzzle velocity is the same, the rate of the revolution of the bullet will be the same. Twist rate simply refers to the pitch of the rifling in turns per distance. Obviously, the faster the bullet is moving, the quicker it's spinning. The twist rate of a barrel is determined based on the minimum spin required to stabilize the longest projectile that's likely to be fired at the muzzle velocity that the gun will reasonably achieve with that projectile. Shorter/lighter bullets will likely be fired at a higher velocity, and therefore a greater rate of spin, and will be stable.

The problem is that it is possible to overstabilize a projectile, and in some cases too fast a twist can damage the projectile because it does not firmly engage the rifling and effectively skids across the lands until it grabs and spins. This is problem with patched roundballs and loose fitting (read easy loading) soft lead conicals. With ML's, because of the extreme range of projectile lengths, types and "typical" velocities, the manufacturers can't easily settle on one twist rate like is usually possible for CF rifles. The 1:66" twist that's optimal for patched RB won't stabilize most conicals or sabot projectiles. A 1:28" twist will stabilize all but the longest sabot and conical bullets, but is way too fast for accuracy with roundballs or "Pennsylvania conicals." The 1:48" twist rate is a compromise meant to provide adequate performance with PRB's and lighter for caliber conical bullets and sabots.

Mike

lemoyne

Greenhill Formula

nchawkeye

The optimum rate of twist is dependent upon projectile length...A longer projectile needs a faster twist...

A .50 caliber inline will usually have a 1-28 inch twist...This is optimum for saboted bullets...

A conical rifle typically has a 1-48 twist...

Now, with round balls, it depends on the bore diameter...
My .40 caliber custom rifle has a 1-48 twist...
My .54 custom has a 1-72 twist...

So as a general rule, a round ball gun has a slower twist...

The other variable is the depth of rifling...A custom made flintlock with a round ball barrel will have cut rifling .012-.015 deep...

A factory made gun will have button rifling, a "star" or "button" is pulled through the barrel to embed the rifling, since it's not cut, it's not as deep...Button rifling usually runs about .006, or half the depth of cut rifling...This is fine for sabots or conicals, but not the best for round ball shooting...