What twist would be right for roundball in a 40 caliber ?

[Kmcmichael]

I might decide what I wanted the gun for. Hunting Squirrels or 25 yard targets might be better served with a faster twist. Mine is 1-48 and it seems to shoot good with two different load. 27.5 and 55 grns of 3F.
I would ask the maker.

 

[ronaldrothb49]

The .40 I had was a 1 in 66. I won more matches with that rifle than any I have owned since.

 

[Boomerang]

So what we are working with here is a combination of diameters, groove depth and twist rates. To get the same surface feet per minute to stabilize a smaller diameter round ball, it has to spin faster in the air than a larger one. If it spins too slow or too fast, it will lose stability. So there is a range of RPM that it has to stay within in order for it to remain stable. The groove depth has to be deep enough to be able to grip the patch and ball at "X" velocity in order to keep it from stripping past the rifling. So for example a .530 round ball exiting the muzzle at 1200 fps from a 1:60 twist is spinning 14400 RPM. A .530 round ball has a circumference of 1.665". So the surface feet per minute (SFM) is 1998. The circumference of a .390 round ball is 1.225". So in order to get 1998 SFM from a .390 round ball at 1200 FPS is to increase the twist rate to about 1:44. Now keep in mind that the SFP increases as your velocity increases and a smaller ball usually has a higher velocity than a larger one. So a 1:48 twist rate should be a good average for a 40 cal.

Now I have a headache!

 

[user 36911]

A deeper groove rifling will generally shoot a heavier load with a PRB. The original Hawken used 1-48 but it was a deep groove. The problem with a lot of 1-48 twist today is they are shallow grove as a compromise to shoot a PRB and conical. Too heavy a powder load in a shallow groove can skip over the grooves and hurt accuracy. A slower twist will also shoot heavy powder loads better. Of course for some odd reason there are exceptions.

 

[Redlemur60]

This might be a dumb question but here goes, does caliber have any bearing on what the twist rate should be for round ball? I assumed that round ball needed a very slow rate, like 1 in 60 or so but seeing a recommendation of 1 in 48 has me a bit confused.

Not dumb at all, barrel length may be more important.than caliber. But my vote for 48or greater 48 you may be able to shoot prb or conicals

 

[1971fx]

Pedersoli scout in .40 caliber. 28 inch barrel twist seems to measure at 1 in 40

 

[LME]

Having 16" and 48" I'd probably end up with an average in between.

1in 48 is the average? If you go much lower than that balls arn't to accurate with normal loads.

 

[smoothshooter]

Man, I know even less than most! How does one go down in caliber? I can see going up from .45 to .50 but, how do you go down to a smaller caliber if the barrel is already bored to 45?

Apologies to the OP for the thread drift!

Have a liner installed in the barrel.