Questions about Spelljammer Adventures in Space

Remember the old DM admonition: if an NPC, such as a Giff, has a flashy weapon or device, then one (or more) of your player characters will somehow grab that weapon or device and claim to know how to use it.

If you do not want firearms floating around in your campaign, then do not let the Giffs, with their affection for loud, flashy firearms, into your campaign. Or, at least redesign the Giff firearm as an uncommon, low power Blast Scepter that recharges each day.

I am not being mean, I am just being careful about setting off an arms race in someone's campaign world.

On the other hand, you could adapt the Artificer in the Eberron setting for the Giff in the Spelljammer setting, in that the Giff firearm is simply a version of Artificer's Artillerist subclass. You will still have player characters with a touch of envy concerning that firearm but that is another matter.

Alternatively, do you want your Spell jamming ship to be outfitted like a steam punk space vessel or like a World War Two era B-17 Flying Fortress bomber that had .50 caliber machine guns literally pointing out of every nook and cranny? (I am not kidding, look up the specifications for a B-17 Model G bomber.)

Late addition: my late father was a fighter pilot in the U.S. Army Air Force during World War Two; he made sure that I knew the planes of the Army Air Force.