The Square Brigadier is a set of horse-and-musket rules based on Bob Cordery’s Chess board game inspired by Joe Morschauser.
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.
Toy Soldiers is a set of free wargames rules for the Horse and Musket period. The author writes:
Abstracted rules for battalion combats in Horse and Musket period. A unit is a battalion. A single player might have 2-5 units under his command but only one commander. The intent of the rules is to allow players to bring a small command to the table of generic troops with no points system or national characteristics. The balance that I am aiming for is that a cavalry unit = an infantry unit = an artillery battery.
Infantry battalions are represented by 6 stands of several miniatures. In 15mm, 4 wide seems to work. Artillery batteries are represented by 3 stands. Limbers should also be included.
Units and commanders will always have at least a single D6 placed behind them. For units, this represents their disorder value (1 is well ordered, 6 being disordered). Commanders will have a Chaos dice which will always be at least 1 (well coordinated command) and may go as high as 6 (Ineffective command control). Often, units will also be marked by a single red die marking their stress. Stress goes from 0 to 6.