Four changes I would make:
1. Some caliber designations are, well, not correct. There's not too many examples of the wrong chambering per se (although some are confusing), but it wouldn't hurt to point out that many of these firearms were offered in other period correct chamberings. This is mostly fluff. They didn't mess it up too bad.
2. The proliferation of double action revolvers in he late 19th century wasn't nearly so rampant as it seems to be in a typical Deadlands game, although Hollywood is nothing to go by either. Most Deadlands games go down in the 1870s.
Colt did make numerous double action revolvers in the late 1870s but the mechanism was unrefined and fragile. For the things people actually used guns for day to day, this made for a liability but they were still fairly popular. Double action model 3s weren't around until the 1880s. The New Army wasn't introduced until 1892, and the Hand Ejector wasn't around until 1896.
Now if you look at the sales numbers though, from 1877 on the Colt Lightning well outsold the Single Action Army (aka the Peacemaker). However there was a long time there where there just so many single actions in private, police, and military hands you were likely to run into one as the other.
The late 1800s and early 1900s (I'd say roughly 1875 - 1925) were a very interesting time in handgun technology. Metal cartridges with smokeless powder replaced blackpowder. It's hard to peg down exactly what happened where, but you had many different technologies running side by side over the space of a few decades. By the late 1910s there were handguns in existence which are very nearly identical to many still produced today, or at least modern handguns owe a lot to those designs. It's not like today where semiautomatic pistols are 90%+ of the market and silly things like single action revolvers are the purview of weirdos like me.
That was long winded. Point is, I'd probably make double action revolvers just a bit more expensive to encourage more PC use of single actions.
3. The notion of a speed load cylinder for many of these weapons is complete nonsense. I'd restrict this item to only working with a appropriate firearms.
4. I forget the fourth thing. It's really minor. Really all these are minor.