Here’s a Horse and Musket DBA Variant — primarily for Napoleonic and Seven Years War, but since the basic principles of war in that era covered a huge number of conflicts, you could adjust as needed.
Rules: 19th Century
Modified Morschauser 19th Century Rules
Bob Cordery offers a set of Modified Morschauser 19th Century Rules. They’re based on the original set by Joseph Morschauser. It’s played on a gridded battlefield
NapNuts Western Gunfight Rules
The Napnuts’ Chew Boon Longe has a set of free wargames rules for western gunfights.
Island Mayhem Pirate Rules
Island Mayhem is a set of rules for playing games in which 25mm – 28mm Pirates are searching for treasure.