Here are my thoughts on guns, these aren't 'official roolz' or anything - just my thoughts about how i'd like to do guns.
AMMO
All guns have ammo, represented by the usage die. Just as it was intended. You roll your ammo die at the end of a combat, and it works as per normal. I'd normally expect usage die's to be in the d8-d10 region.
SPRAY & SLAY
You can bypass the DEX test to hit, do automatic damage but you have to roll the Ammo's usage die immediately.
CRAZY AMMO
High power shells, LAZOR beams, angry bee's - all pack a serious punch. A gun with crazy ammo can roll with Advantage when dealing damage, but must also roll with Disadvantage when roll the Ammo's usage die at the end of the combat.
SNIPERS
Long barreled guns or weapons with computers and fancy shit can skip a turn, 'aiming' and automatically deal damage the next turn. Roll usage die at the end of the combat as per normal.
BOMBS
Area of effect weapons attack 1d4 targets in a Nearby area.
SWISS CHEESE
If you kill a creature with this gun, and have any 'damage left over' you have to give that damage to a Nearby creature. Oh there are only your friends around? Then you shouldn't be playing with guns indoors.
BLUNDERBOLLOCKS
If you have a gun that shoots shrapnel, pellets or other short range spreadables - should you miss, you deal your Level in damage to the target if they're Nearby.
STOPPING POWER
Replace class damage with a d12, the usage die for this gun is always a d6.
That seems like a nice set of rules to make Guns feel distinct, but not overwhelming. Also its unlikely that one single gun would have more than a couple of these little rule - tops. One each also helps distinguish the gun.