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Yes I'd use the 1-38 because the longer twist will work with a short bullet like that, but the faster twist allows for a LOT more leeway on velocities. Revolutions per second around the circumference of the bullet is what gives it stability. (Diameter X pi) The idea is that at a given velocity the RPS around the bullet's middle will be "X" and the slower it goes forward, the slower it revolves. So a faster RPS at a slower speed means you get good stability at low speeds as well as high speeds, but you will find a 1-38 with .004" deep grooves and those grooves a bit wider then lands will grab the bullet well even when you fire it with a large charge of powder. Such a barrel will shoot tight groups with 70 grains of powder and also for 150 grains of powder, as long as the quality is good.

As I said, there are mathematical formulas that give you equations, but in my experience, such figures don't tell the whole story. A man with a rifle often wants to be able to use lesser powder charges and that means slower RPS which can open up the groups. But a faster RPS at a slow speeds hurts nothing.

When I was making my own barrel I did a lot of experimentation on this very subject. I made barrels as fast as 1-26 and fired ball in them and compared accuracy to those I made with 1-76 twist, and I found NO difference at all in accuracy. NONE!

What made a difference was rifling geometry. Wider grooves help a lot. You can't get too narrow for bullet guns so I recommend 1.5 times as wide as the lands for lead to steel fit. In guns that are used with cloth patches I found up to 3 to 1 ratio was very good. Slow twists do help to keep fouling down. A fast twist needs to be wiped a lot more often. But bullet guns need only a quick wipe if loaded from the muzzle. Breach loaders need good lube and a wad to carry lube and soften the fouling, but as long as the fouling is soft one bullet will "squeegee" the fouling from the previous shot.

Just keep in mind, the grooves of the bore become the lands of the bullet. The lands of the bullet need to be strong to resist the force imposed on them without stripping or smearing. Wide grooves in the barrel make wide lands on the bullet and wide lands are MUCH stronger then narrow ones.

The geometry and overall quality of the bore,and smoothness of the tops of the lands make a difference, but contrary to what is written and re-written over and over and over, what I have found to be the least important factor in making barrels for black-powder guns is twist rate. I do NOT say it makes no difference at all, but of all the factors in tool making for the cutting of the rifling, twist was the one at the bottom of the list.
It's on the list, but it's the last thing to worry about.

Edited by szihn (12/02/22 06:09 AM)

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