A visual guide to Gewehr weaponry, as I understand it.
The Gewehr now:
The Colt Single Action Army revolver, chambered in a number of calibers, but most infamously .45 Long Colt. Six round capacity, the hammer had to be manually cocked to fire the weapon each time. Jon's revolver appears to be based heavily on the SAA in terms of function.
The Colt "Walker" revolver, .44 caliber, pre-cartridge technology. The first "magnum" handgun because the larger cylinder allowed more powder to be used (thus increasing the velocity of the bullet) and the frame was heavier and more able to withstand the pressures of added gunpowder. Jon's revolver appears to be based on this revolver in terms of appearance. Like the later SAA, this revolver is single-action in function and must be cocked each time to fire.
Within a few years, twenty at most, the Gewehr can have upgraded to more efficient firearms, like so:
A modern Smith & Wesson Model 586 double-action revolver, .357 Magnum. The most obvious upgrade in technology would be double-action rather than single-action revolvers. With double-action, you no longer have to cock the hammer, then pull the trigger. Pulling the trigger cocks the hammer and drops it in one motion, allowing for a faster rate of fire.
The Mauser auto-loading pistol of 1896. Superior to revolvers in many ways. Instead of holding only six rounds (some revolvers in smaller calibers, such as the Nagant Model 1895, held seven or eight rounds, but most held six) it had a capacity of ten rounds. And unlike most revolvers, which could only be loaded one round at a time (Jon's can be loaded much more quickly since the entire cylinder pops out like modern revolvers), it can be loaded in seconds by placing a clip holding ten cartridges above the magazine and pushing them into the magazine to load it. Every shot was single-action (lighter trigger pull than double-action) because the hammer was automatically cocked when the spent cartridge was ejected and the next one chambered. Later innovations such as replacing the built-in magazine with a removable one (allowing for even faster reloading by simply swapping out magazines, like modern pistols do) and discovering the principles behind fully-automatic fire improved the Mauser c96 even further in 1932.
Of course, Poe's universe has obvious differences. Technology could take a totally different route than ours, especially with the influence of magic. Even Jon's revolver is an amalgamation of firearms from different time periods: the outward appearance of a Walker percussion revolver, the function of a Colt SAA cartridge revolver, and the opening cylinder of a modern revolver for faster reloading.