The .38

Caliber .38 conversions were built on Colt’s Model 1851 and 1861 frames and I’ve seen those. Supposedly .38’s were also built on Colt Model 1862 pocket 5-shooters but I’ve never actually seen one. The .38 Colt chamberings were two: a rimfire and a centerfire. The latter round segued into the .38 Long Colt a couple decades later, by the way.

Here’s an important factor for potential handloaders. Colt .44 cap-and-ball barrels measured 0.451" or more across their grooves and .36’s were 0.375" or more. Therefore .38 and .44 Colt cartridges had to carry bullets to fit those dimensions. This necessitated heel-type bullets where the bullet’s lower part is the same dimension as the cartridge case inside but its full dimension is the same as the case’s outside. Look at a .22 LR for an example. At a later date ammunition factories lengthened the .38 Colt’s case, with the bullet made hollow-based to expand into those oversized rifling grooves — hence the name “.38 Long Colt.” Factory .44 Colts always had heel-base bullets.