I use an adaptation of the FFG "Sorcery & Steam" firearms rules, combined with a little 2e and a little d20 Modern.
Holdout pistol, flintlock: 1d8 damage, crit 20/x3, range 10', 1 shot, reload time 2 full rounds.
Holdout pistol, percussion-cap revolver: 1d8 damage, crit 20/x3, range 10', 4 shots, reload time 8 full rounds.
Holdout pistol, breech-loading revolver: 2d4 damage, crit 20/x3, range 10', 6 shots, reload time 1 full round (Single-action revolvers can be used with Rapid Shot and Manyshot; Double-action revolvers can be used with Rapid Shot and Double-Tap.).
Belt pistol, flintlock or percussion-cap revolver: 1d10 damage, crit 20/x3, range 20'
Belt pistol, breech-loading revolver: 2d6 damage, crit 20/x3, range 20'
Horse pistol, flintlock or percussion-cap revolver: 1d12 damage, crit 20/x3, range 30'
Horse pistol, breech-loading revolver: 2d8 damage, crit 20/x3, range 30'
Musketoon, flintlock or percussion-cap: 2d6 damage, crit 20/x3, range 50'
Carbine rifle, breech-loading: 2d8 damage, crit 20/x3, range 70'
Musket, flintlock or percussion-cap: 2d8 damage, crit 20/x3, range 90'
Rifle, breech-loading: 2d10 damage, crit 20/x3, range 110'
Light blunderbuss, flintlock: 2d8 damage, crit 19-20/x2, range 20', -1 damage each range increment beyond the first
Sawed-off shotgun, breech-loading: 2d10 damage, crit 19-20/x2, range 20', -1 damage each range increment beyond the first
Heavy blunderbuss, flintlock: 2d10 damage, crit 19-20/x2, range 30', -1 damage each range increment beyond the first
Shotgun, breech-loading: 2d12 damage, crit 19-20/x2, range 30', -1 damage each range increment beyond the first