Hm, I think that making this a shotgun with incendiary ammo would work well for this. However that also raises the question of what would the stats would be. These are my stat suggestions.

Pros:
The shells for this shotgun cause people to be set on fire when hit.
Hitting a burning enemy deals minicrits.
In water you do not set people on fire, but you do deal damage.
Cons:
-x% damage. this might be good because if you have good shotgun aim, it is guarantied that you get minicrits. So -15% damage, perhaps?
-X amount of ammo in the gun. 4 total ammo in the mag, maybe?
Close range is the guarantied minicrit zone. Basically standard shotgun effective range.

Good idea, Bad idea? OP, UP? Tell me and no trolling and/or flaming, please. Thank you.
Edit:Put in a stat for decreased damage, specified the minicrit zone.