I acquired not long ago a roller in 43 reformado. It's an original Remington made first contract saddle ring carbine rechambered to 43 reformado (FB on the barrel in front of the receiver). I bought a few 43 spanish brass from Track of the Wolf and some .439 385 grain bullets from Buffalo Arms. I bushed the firing pin and checked the head space. Next I loaded 70gr of FF lightly compressed, a card wad, a 3/16 grease cookie, and another card wad below my bullet. With a string from a safe distance I shot and fire formed 4 cases. The action opened freely the piece of tape I put over the breech and block never moved or showed any signs of blow by and the cases are clean on the outside. I'm confident the gun is safe for black powder loads. The cases fire formed nicely but here's my question and possible problem. The case mouth ID is now .466+/- and the bore is still .440. Can I shoot a .459 bullet or is that to much? In Layman's book he states that most 43 Spanish that were rechambered were not rebored. That's what I have. I've read earlier posts saying to re-barrel it but it's somewhat rare and in good shape I just as soon not. I'm not looking to make it an MOA shooter just a plinker. This is my first shot at BPRC but I have been reloading smokeless for 40 years. Sorry about the long post but I wanted you to have some back ground info.    Neil

A friend had the same sort of issue with an original martini Henry, he sized the brass to the correct inside diameter for the bullets for the bore then made some brass sleeves to fit tightly over the outside of the case neck and machined them to fit the chamber.
Gumpy

Breech seating (which I've never done before) would be an option but the throat is so long the bullet base would be 3/8 inch from case mouth.