Actually, the high damage comes from the fact they're guns. Spycraft 2.0s guns are in the same vein. 7.62x39mm Russian does 3d6, for example, 7mm Remmington and .300 Winchester do 3d6+2, a .410 shotgun slug does 2d6+2, .45 ACP does 1d12 with an effect similar to massive, 5.56x45mm NATO does 4d4. The long load times are because it takes a long time to load a muzzleloader.
The two classic old west cartridges are .45 Colt and .44-40 (which was the most common chambering for the Colt Single Action Army, aka the Peacemaker and the Winchester carbine) do 1d12 and 1d12+2. They also have a quality called Takedown, which is mechanically identical to massive without the size limitation.
As for Incindiary ammo, steal the rules for Armor-Piercing Incindiary ammo from SC2.0. On a critical hit the attacker can spend an extra die to convert the damage from lethal to fire.