The .457 roundball is oversized so that when you load it into the cylinder a thin ring of lead should be sheared off, that is what gives you a good gas-seal because the bullet is pressure cut to fit the hole.
If you're just using powder and ball, you'll want to follow the ball with a seal of grease or bullet lube to make sure no errant sparks from the cylinder you fire can ignite the charges in any of the remaining cylinders causing a chain-fire of all the remaining loads. It's a safety precaution.
Another safety precaution when packing the gun, is to only load 5 of the 6 chambers and to rest the hammer on the empty chamber. This is a very smart thing to do, asseveral of our forefathers soon painfully learned.