I have an Uberti Colt 1851 "Navy" revolver for which I have bought a cartridge conversion cylinder. I know the groove diameter of the bore on this gun is .375 and .38 Special bullets are .357, so I purchased a proper "heeled" bullet mould from LBT so I could make cartridges that were available for these early cartridge conversion revolvers. The bullet casts at 130 grains and .376-.376 with a .357-.358 heel that fits in the trimmed .38 Special cases. Trimming cases so the loaded round does not protrude from the front of the cylinder results in a Black powder capacity of 20 grains of FFFg.
While I am probably going to mainly use real black powder, occasionally I may want to use this revolver at one of our air-conditioned indoor ranges using smokeless powder. At present the cartridge over-all length is 1.425". I am interested in anyone's experience with smokeless powder heeled bullet loads in .38 Colt conversion cylinders. The cylinder manufacturer suggests keeping velocities under 800 fps. I am considering a load of 3.5 grains of Bullseye or 4.0 grains of Unique.