I have a potentially pretty bad habit of adjusting monster/item stats without referring to any official sources on how to do so. I know there are monster templates and item creation guidelines, but I'll admit, I'm lazy, and sometimes it's easier to just say "double the damage and increase the CR by X."
In little over half a year of GM'ing, it's only bit me in the butt once, and a little on-the-spot adjustments fixed things without any lasting harm. For my own conscience though, I thought I'd bring this to the table. These forums have stopped me from doing stupid things before, and I kind of want to test myself to see if I can make something without making something obscenely broken.
On an unrelated note, at lvl 6, the Gunslinger decided he wanted to get a merciful (nonlethal) coat pistol, but the craft check ended up getting it cursed. I don't like cursed weapons except as plot devices, so I figured I would simply make it (to the character's surprise) deal totally lethal damage. I don't want the curse to last forever though, so I decided it would simply malfunction at some point, and the residual magic would dissolve into what I've created below, killing two birds (no healer and player wanting to not kill things) in one stone.
The Item In Question...
Price comparable to a wand of CMW, 30% off for being class specific (am I using this right?)
3150 GP for 50 ammunition.
(50) Curative Cartridges.
Instead of firing a metal ball or bullet, these bullets shoot an invisible magical force that heals whoever it hits. A willing target has an effective touch AC of 10+range increment penalties. This effect heals 1d8+2 damage on the target, or harms undead for the same amount.
Unreliable Ammunition: The magical quality of these bullets are unreliable at best, and only a professional can get them to work right, with a bit of luck. The bullets are currently "dormant" and do not even fire when placed in a gun. By spending one grit point, all the bullets on a gunslinger's possession become 'active' for a number of hours equal to the gunslnger's wisdom modifier (min. 1.) The magical effect can only be used by characters with the grit class feature, but if one character with grit activates it, another character with grit can use it.
Sorry if I've been too long worded (again,) but thanks for those who stuck it out!
On that note...
TL;DR: gun that heals... can it be done?
It can definitely be done, but it's never really going to be an in combat option and more of a spam to full health after a fight kind of deal. Unless you were going to allow the gunslinger to full attack with these like they were normal ammunition?
Comparing the price to a wand of similar power the CMW wand costs 4,500gp and heals 2d8+3 per charge. CMW has a range of touch so the higher healing is offset by the lower range.
A different comparison would be to apply the Reach spell metamagic feat to a wand of CLW. So same as before the wand has a price of 4,500 but now it heals 1d8+3 and has a range of 40ft. 30% off of 4,500 would equal 3,150 or exactly what you calculated using the magic item rules, the only difference being the extra one point of healing.
Although you may want to clarify that the grit that must be spent to 'activate' the bullets is explicitly not a 'deed' from the gunlsinger's class feature else you open the possility of your player taking this down the line.
All the above being said I really don't think that this is going to be an item that causes problems even if your gunslinger doesn't have to make attack rolls and spend grit. Even making it so that he is allowed to spam as many shots as he has attacks is probably fine and actually really cool the more I think about it.
As a side note the magic item creation rules are notoriously wonky and I've found that its almost always better to find an already existing item at least somewhat similar to the one you are trying to make and working backwards instead of figuring out what the price should be according to all of those tables.
EDIT: As balancing features I would:
1) Not allow these cartridges to 'critically hit' because the first thing your players will do after combat is take turns putting the barrel of the gun in their mouth's and pulling the trigger for x4 healing.
2) Be sure that the cartridges don't become an undead slayer's wet dream and allow any undead hit by them a saving throw as per the spell its duplicating. DC would be 12 based on the reach metamagic CLW wand.
As a side note the magic item creation rules are notoriously wonky and I've found that its almost always better to find an already existing item at least somewhat similar to the one you are trying to make and working backwards instead of figuring out what the price should be according to all of those tables.
That would have saved me a lot of trouble XD. I have at least two weeks until our next session, so I thought I'd spend the time needlessly over-complicating things, as long as it's not too easy to abuse.
I think you made the mechanics too complicated. Just make them magic bullets that do cure light wounds when shot at a creature (10 AC for a willing creature).
You priced the bullets wrong. Wands are extremely cheap intentionally because you have to buy them with 50 charges. Also, while you do risk missing and it can only be used by firearm wielders, the bullets allow you to heal at a range. That's a big deal that you haven't factored.
This is a single-use, use-activated item that mimics the effects of a cure light wounds with the Reach metamagic, the equavalent of a 2nd level spell. So a single bullet should be priced at 3*2*50 = 300 gp. Maybe given a small discount because of restrictions and risk of losing the effect. So maybe about 200 or 250.