That is an attempt to bring a bit of variety to the firearms, as well as a dash of realism without ruining the spaghetti-western feel.
Edit: Though there is a little bit of misinformation I see. The Winchester Lever Action 1887 shotgun is listed with an ammo capacity of 4, though the gun in the real world has a magazine of 5 rounds, plus a 6th round in the chamber and you could actually fit a 7th one on the loading arm. Official capacity of 5, typical capacity of 6, and 7 round capacity if you really wanted to push it. No idea why they have 4 listed. 4 isn't even the manufacturer's intended capacity.
Another edit: Just realized Deadlands itself has this gun listed as 4 round capacity. Interesting.
And a final edit: I think the book you're looking for is the Smith & Robards book, published in 1997 for the original Deadlands setting. That's the official supplement for guns and gear, at least. There's also the 1880 Smith & Robards Catalog for Deadlands Reloaded, which is an updated version of the book.
Last edited April 30, 2020 5:00 am