It was a lengthened version of the earlier centerfire .41 Short Colt, which was made to duplicate the dimensions of the even earlier .41 Short[SUP][1][/SUP] rimfire. The front of the bullet was about 0.406–0.408″ OD, the same as the case. The barrel was about 0.404–0.406″ groove diameter. The bullet lubrication was outside the case. At 0.386–0.388″ OD, The base of the bullet was smaller in diameter to fit inside the case. This is known as a "heel-base" or heeled bullet. The only modern heeled bullet is the .22 rimfire.
In the mid-1890s, Colt redesigned the cartridge. They reduced the entire diameter of the bullet to 0.386" OD and lengthened the brass case in order to put both the bullet and its lubrication inside the case. The overall length of both loaded cartridges was about the same. The barrel of the revolver was reduced slightly to match the more popular .38-40 at 0.400–0.401″ groove diameter (this was probably done for manufacturing reasons, not accuracy reasons). This meant that the outside diameter (OD) of the new bullet was smaller than the barrel's bore, let alone its groove diameter. A hollow-base bullet can be dropped down the bore by gravity alone. The newer soft lead bullet was made with a large hollow base, like Civil War Minié balls. The intent was for the base of the bullet to expand with the pressure of the burning gunpowder to grip the rifling...