I will turn the question around a bit. Express bullets are light bullets for a higher initial velocity with black powder. That may not necessarily be what you are interested in with nitro powders and so forth. The question then becomes what bullet length do you want to shoot out of the gun?
The Greenhill formula of 150 diameters squared divided by length equals the twist required to stabilize it (working in inches). Normally the twist rate would be made 1" faster than the formula recommends.
cheers Doug
