OK to reiterate some stuff, and to add some stuff to the discussion, Marine...
The Old School I was taught said:
1:48 twist was a
compromise that shoots moderate loads for round ball, and also conicals well.
If you shoot a heavy load with a 1:48 twist you will strip the patched ball over the rifling and accuracy will be FUBAR
1:60 twist and slower is for round-ball only
WELL..., there are some problems with the above. There is no mention of the
style of rifling. Nor is there a mention of the
depth of rifling.
1:48 twist was highly popular before the conical minnie ball came into regular use, so a lot of folks were using it with round ball and on very large and even dangerous game such as bears. It was found in the 20th century, that with proper width and depth, 1:48 shot both round ball and conical well, so the manufacturers touted it as a "compromise", but historically, it wasn't.
1:72 twist was used in British Enfield rifles shooting conicals during the American Civil War, and those rifles were sometimes used for long range sniping. So much for the idea that 1:60 and slower twist couldn't shoot a conical bullet well....
You will note that in all of the factory made muzzle loading rifle barrels today, for their length, the ball gets at least a 1/2 turn before exiting the barrel.
(This is only important IF you want a barrel that is one length, but the only barrel you can find is too long and you want it shortened.)
The barrel makers slow down their twist rate as the calibers go up; when they are expecting you to use patched round ball. You have a hard time finding a 1:60 twist rate in a stock, factory .40 barrel, for example. You will also note that barrels are also made for larger calibers with a tad faster twist rates than 1:60 for the lads and ladies who want to shoot the conicals.
All of the barrels from the factories are symmetric rifling. Meaning an even number of grooves, and the lands and grooves are evenly spaced, and tend to be .010 depth, give or take .002 or so. Back when rifles were hand cut, often the grooves were thinner, much thinner, so it was easier for a heavy load to "strip" past the rifling. As muzzleloading caught on in the second half of the 20th century, and conicals were popular, some makers made rather shallow depth grooves by comparison to round ball barrels today. This was to ensure the base of the conical bullet, when it upset and grabbed the rifling, it got a good seal as well. So in both cases a heavy patched round ball load could strip the grooves.
Basically, the factory barrel makers have a vested interest in a product that is going to be accurate, and so they know from testing what is going to work, and work well. There are four very popular calibers in rifle barrels, .36, .45, .50, and .54. Whether you choose one of them or one of the slightly less popular calibers, pretty much any barrel made from one of the current manufacturers is going to be a good barrel, for targets and for hunting, using round ball. Some will be good for both round ball and conical.
You only really need to worry about the details when you want a "match" rifle for targets, or when you're doing some very specific shooting like long range silhouettes with paper patched rounds, etc.
Semper FI
LD
