tell him to save his money. Get a Lyman 457125 mold and mold his bullets with pure, or nearly pure lead. First of all, the pure lead doesn't shrink as much as most bullet alloys as it cools. So the 457 may actually throw a 459. Next the soft lead bullet Obturates upon ignition. That means it is hit from behind by the sudden pressure of the burning powder and is "bumped up" in size to fill the bore. 460 is actually kind of small as trapdoors go, for a while Gov't inspectors were told to accept/pass any bore less than 468. I shoot a lot of BPCR, and in a trapdoor, with the Springfield rifling, a pure lead bullet works great. (I've knocked over 9 out of ten 500 meter rams at matches with an 1884 trapdoor, that even has a slightly pitted barrel.)
If your Buddy still decides he needs a larger bullet, to go out a thousandth or two, just cast a hard alloy bullet. Coat it with abrasive and spin it in the mold to make the mold diameter larger. Depending on what abrasive is used, it can be done in less than 15 minutes. including clean up. You are only going out 2 or 3 thou in diamter, which is especially easy in an aluminum mold. There are other ways to take the mold diameter out, but that is the easiest for the home/hobby shooter.
