Siamese Mauser 45-70 issues

[marlinman93]

Well, this is within the scope of vintage....

The new thread area is for vintage scopes, rifle or spotting scopes.

[HumptyDumpty]

The new thread area is for vintage scopes, rifle or spotting scopes.

Just a failed attempt at a little word play on my part, perhaps I should have used purple font.

[405grain]

That case looks like the chamber was cut with a Morse taper reamer. With that much case expansion just ahead of the cartridge head you'd be risking a case head separation from over-working the brass in that region if you resized the brass a few times. Your gunsmith is going to need to evaluate the possibility of setting the barrel back. With that much case expansion the barrel will probably need to be set back more than just one thread. There's usually (but not always) a relief cut between the threaded shank and the shoulder of the barrel. If the barrel needs to be set back too far, and there's a relief cut, there might not be enough threads left on the shank for it to be used safely. If that's the case your gunsmith might have to cut off the shank and turn & thread a new one on the back of the barrel. This would depend on if there's enough meat on the breech end of the barrel to do this.

Though it's technically "tapered", a 45-70 is a straight walled case. Unless you make up a custom die for it, "neck sizing" is out of the question. To "neck size" a 45-70 case you'd have to full length resize it. This is not a good idea because of the possibility of creating a case head separation as stated above. Also, rechambering to 45-90 will not clean up this bulge because both cases have the same size case head. Your best solution to this problem is to set the barrel back and re-chamber. Your gunsmith will have to decide if that's possible. If it isn't, then a new barrel would be your second choice.