Back again with more questions and less answers, I play a Musket Master gunslinger in a campaign where modern firearms were introduced into the story at level 8. When building my character I had talked with the DM about building around a double hackbut, but due to the way the story has progressed it isn't the easiest weapon to deal with on top of having to juggle a secondary ammunition type while crafting.
I had asked my DM about using the same type of ammunition for all my rifle based guns thus modernizing the hackbut but he is stuck on the aspect of cartridge type ammunition loading faster and doesn't want any other builds to have access to such a powerful weapon and have it fire fast due to the mechanics. I understand his hesitation and have been trying to think of a method which would satisfy both problems
What I came up with is designing a custom breach loading mechanism where an amount of gunpowder (since I already carry a powder horn) would rest behind the chambered shell and when the firing pin fires the shell, the gunpowder would also ignite and the extra expanding gas in the chamber would result in a higher shell velocity, as well as eject the casing.
Since this is a fairly silly idea I doubt any guns from history would use this mechanism, but it would explain why the same shell gets double the damage dice as well as solve the fast reload problem since there would be an extra step as well as a complicated locking mechanism to contain the forces.
is this completely far-fetched or is it a viable solution to both arguments?
From where I'm standing,
A metal cartridge has a base cost of 15 gp, and can be loaded as a move action (free with rapid reload). I might increase the time if you attempt to muzzle-load a longer barrel unless you're good at loading longer guns specifically
A paper alchemical cartridge has a base cost of 12gp, and can be loaded as a move action (free with rapid reload). As a musket master, this applies to 2h guns in addition to 1h guns. It also increases the misfire by 1.
So a (one-chamber) gun loaded with metal carts but has +1 misfire is very slightly worse than the same gun loaded with paper carts and normal misfire rate (since the ammo costs slightly more, but otherwise the weapons are identical)
So I would just say that whatever you use to mod the gun increases the misfire chance by 1.