Full information about the mod can be found on the Minecraft Forums at http://www.minecraftforum.net/topic/1676566-162-forge-smp-flintlock-weapons-mod-v16-4-100000-downloads-now-with-mounted-cannons-and-finally-16/
This mod adds ten new flintlock weapons. Loading these flintlock weapons in real life is quite cumbersome, and this mod replicates that. To load a weapon, you must craft together an unloaded one, gunpowder, and your ammo. The multi-shot weapons can do this multiple times to become fully loaded. Also be aware, your gun may misfire, making a sad click and losing your ammo. Reload and try again. Your guns can be repaired at an anvil with iron ingots (but don't try renaming them).
You can customize your gun's look and stats by adding inlays. Craft your unloaded gun in the middle with a diamond, gold ingot, emerald, or obsidian block on all four sides of it to inlay it with that material. Gold prevents misfires, diamond increases durability, emerald boosts range, and obsidian helps all three slightly. You can use this feature to fine-tune your gun to better fit your needs, i.e. if you like to pick those creepers off from afar, you might want to inlay your musket to boost its effective range. Or maybe you're planning a long nether trip and don't want to lug around an anvil to keep your revolver in working order, then inlay it to increase its durability.
A config file can be found in /.minecraft/config/flintlock_guns. You can adjust item/block IDs (set a number to start and the mod will use IDs counting up from that, so you don't have to change a hundred IDs manually) as well as set a global damage multiplier if you need to balance the mod to your needs. You can also change whether or not explosive weapons will destroy blocks when they explode by setting BlockDamageOn to true or false.