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?
Some quick thoughts, which may or may not be helpful from a gameplay perspective but should be considered from a world perspective:
The popularity of rifling is only indirectly a result of smokeless powder (or your “force powder” analogy). What smokeless powder really permitted was for reliable and effective metal cartridges to be created. Prior to this, most rifles were muzzle loaded, and you had to physically screw the projectile down the barrel - a laborious and time-consuming task, and what made them actually unpopular for military use. (In-game, this might be represented by rifles having to take an extra turn to reload, but having additional range or a bonus modifier to hit.)
Making an effective breechloading rifle is highly dependent on also being able to reliably produce a metal cartridge that will always fit in the breech, reliably fire, and the bullet fit the rifling no matter where the cartridge or bullet were made. This implies certain metalworking and machining techniques which may have an impact on your world.
One thing I don’t see mentioned is the primer. In a metal cartridge, the propellant isn’t itself detonated by the hammer, but a more-reactive material called the primer is. By definition, the primer is unstable enough that it is unsuitable as a propellant; likewise, the propellant is stable enough that it is unsuitable as a primer. (In weapons prior to modern all-in-one cartridges, the primer sat in a small metal cap which was fitted over a nipple that lead into the firing chamber of the gun; the cap would be replaced after each shot.)
Assuming it’s a close analogy of smokeless powder, Force Powder might not be a great primer. That said, your world may have invented the means to produce a viable chemical primer, such as mercury or silver fulminate, which will detonate a main propellant charge of Force Powder.