Cat: So, we’re making a setting for a slightly home-brewed Pathfinder 1e campaign. We want guns in the campaign, but the official firearms rules are irksome.
Cat: Anyway, as one of the changes we’ve made, we’ve switched propellants to a technically magical explosive called Force Powder. Force powder is smokeless, does not foul the workings of a gun, operates just fine in water, and can only be set off with a sharp percussive force; examples include smashing it against a metal surface with a hammer, or an explosive shockwave.
Cat; Among other things, we’ve figured that this will probably make rifled firearms a fair bit more popular early on than in real life; one of the big reasons they weren’t widely adopted for military use is because all the smoke quickly nullified their range and accuracy advantages. It also means that matchlocks, wheel locks, and flint locks basically don’t exist, with only percussive ignition being feasible.
Cat: So, are there any other things we should be aware of?
Lucifer: We’re uncertain of the effects on breech-loading firearm adoption. It seems like it should help?
I would suggest thinking of ways that your setting might try to control these weapons. Imagine being a high-level magic user in charge of security - what could you do to create safe zones? Could you work out a spell to passively transmute force powder into an inert substance? Could you cause magazines and powder stores to ignite from afar?
Further down the line, how would people try to get around these countermeasures? It’s always fun to run a mile or two on the measures-countermeasures treadmill and see where it takes you.