Recently, after one of our PC's fell off a mountain we welcomed a new fighter to our party, hailing from a far-off land.
Our party is all level 3. I'm playing a dwarf artillerist artificer. My character is the only person (that we know of) to have invented firearms. Given their novelty and effectiveness of my weapons our new fighter PC has taken interest in my inventions, and after saving me from a particularly nasty encounter I decided to gift him my backup pistol as a token of thanks.
However, this raises the question:
Does our fighter have proficiency with the firearm I just gave him?
The PHB lists fighters having proficiency with all simple and martial weapons. So by the book he would have proficiency the moment he starts using it. Understandably, however my DM has ruled them to be "Too New" and that I will have to train with our fighter for some time before he has proficiency.
What would be a good way to quantify training with such an item? How long would it take for our fighter to learn how to use this gun? Are there any benefits that me being an artificer would bring to the table?
For reference, I gave him a palm pistol that uses these stats.
The damage isn't high enough to outclass any of his other abilities (he's a samurai lol) but outside the game he does want some ranged capacity.