Out of the cartridges you listed....I’d run the 6BR. Barrel life will be so much better than the .243win., 6mm Rem, 6CM etc...I’ve shot them all. Also vs. the 6 Dasher you don’t have to form cases at all. To me the Dasher can be temperamental with that really short neck but it is a shooter. The 6PPC would most likely need a different bolt head. The rim diameter is smaller than the others. It might not but I think it would make life easier.

I’m shooting a 6BRA right now on a rebarreled Savage 112 J series single shot. 107smk are coming out just under 2900fps and that’s out of a 26” barrel. You just shoot normal 6BR loads thru it and no difference in accuracy vs. the formed 6BRA case.

Later, Frank
Bartlein Barrels