Most jags (sealed bags on hook racks) in the stores I've seen, have brass threaded sections. Some, not designed for muzzleloaders have tapered pointed ends out the front
for piercing patches. I just snip those off and file the ends flat, then cut off the threaded protrusion, drill and tap for a section of machine screw in 8x32 or 10x32, insert one end
of a cut off screw with a drop of CA and it's done.
I've seen sizes on the racks from .30 cal. to 12 bore, even including .50, .54, .58, then 20bore etc in the smoothbores.
I use a slightly turned down 16 bore jag for my .69 (10x32 screw), for using doubled flannelette.