Please post any thoughts or opinions you might have
What's your goal?
From the description you've given, why would a caster choose to use this over a standard rechargeable wand or staff, or ring of spell storing?
What's the benefit of using physical cartridges over spell charges? I'm not familiar with your game world, as you haven't mentioned it, but in mine it's standard practice for magic missile wands, fireball staves, etc. to be able to easily take new charges by simply casting a memorized spell with intent to recharge rather than discharge. A standard ring of spell storing can accommodate a number of spell levels of any spells the mage wishes to imbue it with. So if you want to have 50 fireballs on hand, you charge your staff over a few weeks. If you need a variety of spells, you put it in the spell storing ring. Again, these are totally generic, standard issue random-drop items. An enterprising mage who wants to create their own items can combine these properties to get a swiss-army knife effect of high capacity spell storage.
What's the benefit of creating a spellgun that does this same thing, except that it requires that rounds be individually created through a special process, but nevertheless can explore of backfire on the user?
Actual casters shouldn’t need to use them
...ahh, so the entire point of these is to give high-capacity ring of spell storing type items to non-casters? This immediately brings up questions of balance.
offensive-type spell cartridges are too precious to be
used as a general replacement for bullets.
...ok, but if bullets are readily available, then for it to make sense to have spellgun bullets at all, those spellgun bullets will presumably have to be significantly more powerful than bullets, yes? What's the benefit of making or using a rare and precious spellgun bullet that requires lots of time/magic/gold/etc to acquire, and has a chance of backfiring on the user, when things like magic arrows, bullets, and mages who can cast the same spell with no chance of backfire, are all readily available?
Examine the economy of this device. Does it make sense to build one?
sort of Swiss Army knife with interchangeable tools
Ok. Then this isn't really a "gun" then, right? No reason to limit the rechargables to offensive spells. Although it does bring into question the nature of the mental/magical "targeting" that the device might use. A caster can direct a charged spell being discharged to its intended target. A non-caster generally wouldn't know how.
Making these telepathic, sentient ego items / artifacts would probably be the simplest solution., so that they can receive mental direction from the user regardless of casting ability.
Please post any thoughts or opinions you might have.
What you're describing could certainly be created, at least using the standard item enchantment rules from my campaigns, but it seems like exactly the sort of thing that mages would be reluctant to make. They would be fairly time consuming to make, and would give away substantial magic power to non-mages. Though the single-use rounds brings up interesting possibilities for highly lucrative arms monopolies. Even so, I would think these things would make a user about as high priority a target for mage guilds as a typical mageslayer weapon wielder.
But again, campaigns vary.
In any case, if mages are maing these things with intent to create monopolies, or perhaps to alter the nature of war, then why would they choose to make the reachargable rounds physical? Wouldn't it be vastly easier and more in their interest to simply make them able to receive standard spell charges? Adding/changing the stored spells would require a mage to do it, rather than making it, and allowing the user to change rounds as desired, but from the perspective of enchanters, again, it seems like it would be less effort for them while simultaneously giving them greater control over the result.
So again, what's your goal? And what's the goal of the enchanters who are making these things?