Alright, so basically, I had the idea for this magic item last night, and I did a quick write-up so I could start fine-tuning the balance. Feedback on the mechanics would be greatly appreciated, as I do plan to make this item available to my players in my home games!! With that said, here's the (unpolished) description I've got so far: 

• Rarity: ??? (I need help with this one, lol. I'm thinking either Uncommon or Rare??) 
• Requires attunement?: Yes. As part of attuning to the gem, you attach it to one firearm with which you have proficiency. Thereafter, the firearm gains the properties specified below. A firearm can only have one gem attached to it at any one time. As part of a short rest, you may remove the gem from one firearm and attach it to another firearm with which you are also proficient. 
• Properties: 
  • The gem creates ammo for your attacks if the firearm is empty, allowing you to ignore the loading property of that firearm. 
  • This ammo is magical, and when you attack with it, you replace the damage type of your attack with force. 
  • Charges: The gem also has 5 charges and regains 1d4 + 1 expended charges each dawn. Before you make an attack with the weapon the gem is attached to, you may expend 1 charge to alter the nature of the attack. Instead of dealing damage on a hit, the target must make a con save with a dc of 8 + your prof + your dex or fall unconscious for 1 minute. 
• Variants (probably the same rarity as the standard amethyst): 
  • Diamond: The ammo deals radiant damage rather than force. You may spend 1 charge to shoot a willing creature as one of your attacks (no attack roll required) and heal it for a # of hp equal to the damage you would've done had you attacked as normal. The diamond is worth 500 gp and can be used as a material component for resurrection spells, though doing so ends your attunement and destroys the gem. 
  • Onyx: The ammo deals necrotic damage rather than force. Before you make an attack, you may spend 1 charge. If you hit, the target takes an extra die of damage, and you regain hp equal to half the damage dealt with this attack. 
  • Sapphire: The ammo deals psychic damage rather than force. Before you make an attack, you may spend 1 charge. If you hit, the target must make an int save with a dc of 8 + prof + dex or have disadvantage on the next roll it makes. 

I have a couple notes I'd like to share on my reasoning behind the item's creation before I turn the description over to y'all for feedback!! 

• First, purpose: Honestly, I had the thought of "wouldn't it be cool if there was something like the Ruby of the War Mage but for guns?" and went from there. But also, I really really really want to run Out of the Abyss in college, and for lore reasons, firearms in my world are a relatively rare invention of the surface-dwelling dwarves, and it simply wouldn't be feasible for a gun-toting character to purchase ammo in the Underdark. But I also don't want a player to feel penalized for playing a character they think would be cool, hence the ability of the gem to create magic ammo (that also helps with overcoming resistances!) while attached to a firearm. 
• I'm still wavering on whether allowing a character to ignore the reloading property of the firearm the gem is attached to unbalances firearms too much, but there are a couple things pushing me in favor of keeping that property of the gem. First, it makes logical sense for a gem whose purpose is to create magic ammo! And second, there are other ways in the game for a character to bypass loading as early as first level--the Gunner feat, the Artificer's Repeating Shot infusion, and the Giff race are the ones I can think of off the top of my head. Also, Crossbow Expert. My point is that there's a precedent for this kind of allowance, so I personally think it'll probably be fine. 
• Regarding the extra charge properties (aka the stun setting): I acknowledge that being able to knock an enemy unconscious for 1 minute is powerful. Some might call it broken. However, there are a couple features built into the design of this property of the standard gem that I think balances the strength of this ability. First, you spend a charge before making an attack roll, so you could miss. Second, it targets con saves, which tend to be strong for most monsters. Third, unconscious creatures also fall prone, which means that if the knockout shot lands, you'll most often have disadvantage against any further attacks you make against that creature. Melee combatants would have advantage, but hey, that just encourages teamwork :) Also, it could be used for out-of-combat utility! It's situational, but D&D is all about situations, and also, the martial characters who would benefit most from this item (i.e. Fighters) are often widely regarded as lacking in utility options anyways. Also, it's a good way to help burn through legendary resistances, which, again--teamwork! Huzzah! 

Okay, that's about all I have to say for now!! I would love to get some feedback before I write up a polished version that I can share with my players, and maybe also with the community, if there's interest. Thanks in advance!! :) 

I acknowledge that being able to knock an enemy unconscious for 1 minute is powerful. Some might call it broken.

Yeah, I'm one of those. Your melee buddies don't just have advantage, they are auto-critting every hit on an unconscious enemy. Just instantly obliterating or permanently banishing the enemy instead would actually be less powerful because you couldn't use it for capturing or interrogation.

What you have created here is a "save or suck" mechanic, which D&D has been moving away from ever since 1e. In general, the design choice of "very powerful but unlikely to happen" features have become much less common because 1) there are often ways to min-max the chance of success to end up with a success rate substantially better than the designer intended and 2) your cool feature failing most of the time is just a bummer. I have seen players abandon a thing completely because "it never works anyway." It's just much more fulfilling to have something that reliably grants a moderate benefit.

If it's supposed to be a stun setting, then maybe it should give them the stunned condition instead. Or these special bullets could deal nonlethal damage, and if they dropped the target below 0hp it would be stabilized but unconscious. There are ways to provide the utility you're looking for without having the whole fight hinge on one saving throw. Or heck - make it 100% utility by encasing them in a Resilient Sphere or the like so that this would only be useful when you want to keep your opponent alive.

As for rarity, force damage and removing the loading property alone puts it into Rare territory by my estimation. The charges having significant utility (even with the knock-out bullets being scaled back) probably would bump it to Very Rare.

Quote from scatterbraind >>

Your melee buddies don't just have advantage, they are auto-critting every hit on an unconscious enemy. 

Oh, whoa, I totally missed that when I checked my dm screen last night (weird that that specific part of being unconscious wasn't listed on there). Yeah, stunned seems safer to make available. Do you think a repeated save at the end of each of the target's turns to end the condition early (in the vein of spells like Hold Person) would temper the feature's power better? 

Thanks for the feedback!! 

Quote from ImmortalArizona >>

Quote from scatterbraind >>

Your melee buddies don't just have advantage, they are auto-critting every hit on an unconscious enemy. 

Oh, whoa, I totally missed that when I checked my dm screen last night (weird that that specific part of being unconscious wasn't listed on there). Yeah, stunned seems safer to make available. Do you think a repeated save at the end of each of the target's turns to end the condition early (in the vein of spells like Hold Person) would temper the feature's power better? 

Thanks for the feedback!! 

 Yup, any condition that incapacitates an enemy for 1 minute without a save or other easy way to remove it might as well be "the target instantly dies on a failure". Consider blindness or frightened as an alternative, even Paralyzed is less debilitating that stun because there are many ways to have immunity to it.

Sounds good!

Guns in DnD are, on average, more powerful than similar ranged weapons. The absolute weakest gun listed in the DMG deals 1d10 piercing damage... the equivalent of a Heavy Crossbow. It's meant to be balanced by the loading property and also the fact that the ammo is generally more expensive, if not something that your character has to build themselves in their downtime. Just having a weapon that provides its own ammo is a pretty cool feature, but it's also something an Artificer can do pretty early on, and they get a +1 to attacks/damage on top of it.

I'd prefer to have something more reliable, even if it's less potent. You already need to hit your opponent with an attack in order to trigger the secondary features of this item, and then they get a very common save to resist on top of that? It'd be amazing when it lands, but I'd rather have something lesser that happens automatically... like, if you spend a charge then hit your target their speed is reduced to 0 until your next turn. They still have all their other actions but it stuns them in place... good enough to be worth a charge, but not good enough to justify asking for an extra saving throw.

For the Diamond... I think instead of just saying that it could theoretically be used as a component for revival spells, you should allow it to be used to actually just cast Raise Dead, but doing so destroys the gem. Losing your Very Rare magic weapon is such a big loss that I think it justifies the additional potency of not requiring you to give it to someone else just to use as a component.