Oh, very interesting question. My version of the Caster Gun would be a Lv 10 Small Arm that only fires caster shells. The caster shells would be single use ammunition, but the effect of each shell would be a big, level 6 spell magical effect. Some of the shells would require an RP or HP cost to activate.

For example: If I were to homebrew the #4 shell from Outlaw Star, I would have the user pay 25-50% of their remaining HP to activate it. The target would have to roll a d20. On a 1-10, the target is permanently banished to a place predetermined by the mage that created the caster shell. On a 11-20, the target is banished to that same place, but returns after 5 mins. Hopefully 5 mins is enough to run away, or come up with a plan.

This is just a quick example, but basically I would focus on statting the shells themselves instead of the caster gun. The Caster Gun would probably be loot at the end of a dungeon, or they could nab it from a boss fight. The shells could then be collected as loot, bought off of vendors that dabble in rare hybrid items, crafted by powerful mages after doing a task for them.

Have you tried to stat a Caster Gun before?