To use the caster, alchemy is probably the better model, but unfortunately, alchemy shelf lives are so ungodly terrible as to make it highly impractical. In this case, the gun is more or less irrelevant; it's just an aesthetic means of conveying a spell effect. Instead of loading bullets, you load alchemically prepared spells.
Note: Actually preparing bullets alchemically to hit with both bullet damage and spell damage, possibly multiple cases of spell damage, doesn't really work, cool as the idea is. Firing the gun ruins the spell. The official ruling from Street Grimoire is that while a lot of folks have tried it, no one's gotten it to work past ball musket, and even then not very well. So these would be caster shells, not magic bullets.