Looks a bit... wildly imbalanced.
Let me see if I got it right.
The amount of shots fired is 1d12 + STR mod and each of those deals 1d10?
That gives you a minimum of 1 and a maximum of 120 DMG. And that's just assuming that your SRT modifier is 0.
Personally, I don't see what's the use for this as a siege weapon. If you can use it on regular encounters, then it's vastly overpowered (I assume that it uses touch attacks, like most firearms, but there is no range). Purely as a siege weapon it has little to no appeal.
Now for my suggestion: just make it a rifle that fires 2d3 times per round for 1d8. I would add a missfire value, probably that of the rifle too. Also, you should set it up on a tripod or something. Meaning that it eats up actions on combat and it's mostly used to set up defensive positions.
Well, I never considered making it a personal weapon, let's try to imagine that.
But lets remember, my party is not made up of unrealistic comic book characters, so that's definitely a no-go.
This weapon is surely not for personal use, as Derren pointed out, it is usually horse drawn, it's heavy weaponry, it's a
Siege weapon, I made it for the times your characters are going to be involved with the military or a personal army. Characters don't have personal ballistas, and if they did, they wouldn't take them around everywhere, because its heavy and hard to move. The gatling gun is not for taking down the dungeon boss in the depths of a cave, its for taking down a legion of orcs (lets say 90-150, I'd treat them as one large piece on the board though just to make it easier to handle, but add the health together, so it could range around 540HP for this legion as a whole ) attacking a settlement. Most gatling guns my party would come across would be stationary, attached to the side of a boat like a swivel, or attached to a carriage used by the Dwarf confederacy or something. If a character somehow convinced me to let him own a gatling gun, then he would have to worry about dragging it by horse or carriage everywhere, meaning it would probably be left home a lot, and he certainly wouldn't take it down into a cave or something of the sorts. Next, he would have to worry about his own personal ammunition, now I didn't put it down on this list mainly because you normally don't own yours, you borrow it from an army. If you owned yours, you would have to but each bullet, and seeing it fires 12 plus your str modifier each round, he probably couldn't afford to take it out everyday, stick with your revolvers dude.
Let's also be aware, they'd probably only see these at later levels, this certainly isn't level one gear. He's not going to use this on a village of goblins, he's going to be using this on the back of a carriage shooting a herd of tyrannosaurus's (each having 153 hp, maybe more or less) that are chasing you down. I would also probably adjust the XP gain while using one of these as a means to mow down foe's,
Characters using the gatling gun gain 1/4 experience while defeating creatures and vehicles that you would normally gain full xp from. Maybe I'll adjust it to 1/5 or 1/7 or something, have to do the math first.
