re: many people's gun arguments, not going to quote them all

Gunpowder already exists in D&D, or an equivalent at least (tindertwigs). Problem solved. If a person in the real world can handload a cartridge that will cycle an autoloading pistol with strike-anywhere matches or snap caps, you can be very certain that a sufficiently skilled alchemist will be able to reproduce at least that with alchemy checks and some time. If you put the idea in the head of an alchemist who can manage a DC25 craft check you'll be producing smokeless powder via alchemical means in a year or two (given time for experiments and such). If you happen to know about the interactions between nitric acid and cellulose, you can probably cut that down to a few months. Once the process is known, you can outsource it to other alchemists.

Firearms don't contain copper parts generally. Literally all the components of a gun can be made out of steel. Wood furniture is not needed to make a gun; skeletal stocks made of steel have been around since mass production was invented. If you want to be extremely technical, yes you do need different kinds of steel; specifically spring steel for the sear and buffer/slide springs, hardened steel for the parts of the action subject to wear (slide rails, bolt face, firing pin, hammer, barrel, etc.) and any cheap steel for the frame and the other parts. In practice you could still do it in one cast though, since you could use fabricate to replicate the process used to make different kinds of steel (since most of it is heat treatment and different kinds of forging).

Cartridge cases you need brass (a copper/tin alloy), but bronze has already been discovered if steel objects exist, so just buy the bronze and fabricate it to make the cases. You can make steel cases too if that's a problem, Russia already does that and steel is more common than bronze.

Bullets you just need lead alloy, ideally lead + bismuth or lead + antimony. You could make them with pure lead, but that's really soft and not super ideal. You'll probably create a market for lead overnight, since it's much more common than the other metals you want and not useful for much. Have the lead alloyed before it gets to you and fabricate it into bullets once you get it. Better yet: make steel bullet molds and have someone else do it. Make sure that you discuss the issues of working lead with any potential workers, and have some form of toxin resistance available to smiths who are smelting it.

Loading simple cartridges is something you could mass produce with unseen servants or animated skeletons. It's pretty simple stuff; fit a primer (made by alchemists, essentially a tindertwig) into the cartridge case, put (measured) gunpowder in the case, double check the powder charge to make sure it's the right amount, then seat the bullet into the case mouth and crimp the case (which would require you to make a case crimping device, also not a very hard task). If anyone reading this thread wanted to learn to do it, it would take less than a minute of explanation at each step to show someone how. It's really that easy to do.

Fabricating Sten guns or an equivalent would not require a very difficult Craft check (with Fabricate), and producing rudimentary cartridge-fed breechloading smoothbore guns would be even easier. Combine that with oils of magic weapon or a custom spell similar to Flame Arrows (sor/wiz3) that worked on bullets...