Search:

pretty sure you can buy...or have made...an adaptor to fire it through a .308 or larger .30calibre case.....adaptor is basically a solid .308 case with chamber cut inside rear of it.....
would take...

so....you talking lead/throat.... well to my simple non engineering mind you garden variety drill bit has what 45 degree angle ,you run correct size in to remove the rifling to required depth...then...

would M1 carbine round do same for you...might be more main stream so easier to source.
if straight walled design....it can be done with boring bar...well it can with .45/70 anyway,so if you clever...