Hey Guys
I'm a newbee here, just joined. I have a 45/410 Super comanche pistol that i am working on too. A few others have tried this before. If you do a search for '450 mongo' on google you can find some info.
So far i use Remington 444 cases, expand with 45lc dies, use cci lp primer, and 15 - 18 grains of 2400 powder and either a 230 or 250 grain cast bullet. I think your rifles are stronger than this pistol and i am trying to stay below 14,000 psi loads, just like a 45 colt load. I've called alliant and hodgdon looking for load info and pressures but no one will advise due to legal responsibly. I quit at 18.9 grains of 24oo because it 'feels' like a 3' 410 shell.
The reason for the taper in your rifle is so you cannot load a 454 casull or 460 s&w cartridge into the 45/410 barrel. The 45 and 410s operate at 12,500 to 14,000 psi. The 454 and 460 run around 50 - 60,000 psi. Now, if you will never get rid of the gun and know that you won't try those 2 cartridges, the thing to do is to polish the chamber using emery cloth on a spindle with a die grinder to open up the chamber all the way to the forcing cone. It is very easy to do because you are only removing about 10 -15/000 (.015") of an inch.
But after doing this, a 454 and a 460 will also chamber, which may not be a problem in a rifle that you can buy in that caliber; but if it won't handle that kind of pressure, IT COULD BE DEADLY FOR THE SHOOTER>
Be careful and have fun, I am.