The .41 Long Colt cartridge was created in 1877 for Colt's double-action "Thunderer" revolver. 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 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.