I think I finally sorted out Caster Guns for my world.

Hand Casters = usually single shot hand guns that fire spell shells.
Skilled arcane artist make heavy use of on the fly spell modifiers somewhat similar to sorcerer meta magic. Also makes a great side arm in general.

Mult-casters = shotgun sized that holds several spell shells with a manual cycling mechanism
Users are less likely to use on the fly spell modifiers and tend to be far richer due to the cost of shells. Sometimes comes enchanted/engraved with a single spell modifier, but really it's main advantages is blasting usually the same spell over and over as quickly as mechanically operating the weapon. Such as working the pump, leaver, bolt, etc.

"Tumpers" (haven't thought of a more official name for them) = personal grenade launcher type weapons.
They use extra large spell shells that almost always have spells with area of effect or explosive effects, with the disadvantage of range, cost, and difficulty in making spell shells

And lastly
Longcasters = long range rifles
Uses very specialized spell shells and often has many built in adjustable enchantments/engravings and spell modifiers as well as arcane artist being able to use their own spell modifiers on top of the enchants. Also capable of pinpoint accuracy at extreme ranges.
Main downside is it takes a extremely skilled user to make full use of it, the longcasters are themselves extremely expensive/hard to build, and the spell shell casings are harder to get compared to other spell shells which are mostly standardized.

Might add in some sort of equivalent to grenade that isn't just called a spell grenade or spell bomb. Maybe witch bottle, hex capsule. It don't know. Hasn't come up in story yet.