that's a bit of a problem, and one reason why firearms are bit loopy for D&D rules. Muzzleloaders were distinctly difficult to load and fire (particularly for matchlocks, which are rather unsuitable for a typical 'adventurers going into a dungeon' setting).... without training on how to do it, you can't really do it. OTOH, old firearms don't take years of training either, you can become proficient with them way faster than a sword. Maybe an easy shortcut would be 'have to have proficiency with firearms to use them'? I'm not very familiar with the 3E and later rules, so not really sure.