If you are going to try to match a modern bullet of great weight, such as you have suggested with the 405 .45-70 lead bullet, It would need to be sized down the bore diameter, or smaller and then paper patched, and you really need a faster twist barrel to get the best accuracy. The standard ROT for the .45-70 is 1:22 ". But, target barrels often come in 1:18, or 1:16" ROT. The faster twist barrels can shoot the heavier, and longer .45 cal. bullets, up to 550 grains.
There is nothing wrong with using your existing barrel with RB to take deer. I have seen deer killed out to 100 yards with these guns. The point blank range for the .45, zeroed either at 100 or 110 yards, lets you hit between 3 inches high at 60 yds, and 3 inches low at 130 yds. You, nor I, can hold better than 3 inches using open sights at 100 yds., especially shooting off-hand. You can work out lighter loads to hit POA at 25 or 50 yards for varmints, squirrels and coyote. Assuming you have enough barrel length to burn it, try 60-65 grains of FFFg powder behind a .440 PRB with a .015" pillow ticking patch for the long range shooting. Use a powder charge down around 35 grains of FFFg for the short yardage shooting. Its more than fast enough to kill any small game, and it will dump a coyote out to 70 yards right quick, too. Go for the head shots when you can.
Reminder: You are not shooting a modern bullet. Round Balls work a totally different way. You are starting out with a ball that is as big in diameter as most .30 caliber and smaller bullets expand to. It kills by expanding quickly to 60 caliber and larger in small game, that don't have bones to expand those bullets. The primary wound channel leaves a huge hole compared to modern bullets, and the animals are killed by shock, and internal hemorrhaging that causes a sudden drop in blood pressure, and unconsciousness from lack of oxygen to the brain. Modern bullets are designed to kill by shock alone. Its accurate placement of a RB that will kill for you in a ML rifle. Worry less about velocity, and more about accurate ball placement and you will take home a lot of game. Inside 50 yards, where most deer are actually shot, a .45 may pass complete through both sides of a deer on a broadside shot. You don't need any conical to kill deer, if you are shooting open sights, and are a good enough hunter to put yourself within 100 yards or less of your quarry. The recoil of conicals can make a flincher out of a potentially good shot. Too many people think you have to send a conical out a MLer at the same kind of velocities that they get with their .25-.30 caliber deer rifles. That just is not the fact with MLers.
