True, the original non-magical hand cannon would be a exotic weapon at least!
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... why? It took years to master the bow. Learning how to use a hand cannon took days at most.
Perhaps exotic in the sense that they are quite rare?
I'd need some more information. What, exactly, do you want this hand cannon to do? Are we talking about a standard pistol? If so, from what era? Single shot? Black powder? Do you want it to have the potential to out perform conventional D&D weapons, or is it just an interesting missile weapon?
For what it's worth, I had a PC discover an ancient artifact peacemaker in a campaign once. It did 1d10 damage, and on a roll of 10 you got to roll again, and again for every roll of 10 thereafter. Bullets were scarce and magical and required the blood of albino gnomes and could only be crafted by a particularly mean spirited alchemist.
Literally a Chinese style hand cannon.
I'd probably have it take an action to use for a single shot (no extra attacks with other weapons), do 8d6 in a 120' line (like lightning bolt, but all bludgeoning/piercing), and take 2 actions to reload it. Also explodes on a natural 1 (8d6 piercing damage in 20' radius centered on the user).
Magical ammo could create a stinking cloud at the end of the line (or centered on the first creature hit in the line), or could explode with each creature it hits, causing smaller AOE's as it passes through, or could "bounce" allowing you to send it around corners.