I wasn't even thinking type of gun. Simply putting an enchanted slug that close to explosives (powder and primer both) would be plenty dangerous. Yeah, a loading machine and the explosives aren't exactly cutting edge tech but even minor mishaps can kill you when you a hopper full of powder right in front of your face.
Could it be done with tech-fouling mortal magic? Probably. Regularly without getting your face blown off? Much less likely.
Marcone does it in Even Hand. Uses a magic bullet enchanted by Gard to kill a Fomor cantrev. I don't see a problem here.
It'd be best to stat magic bullets as potions; they're one-use, usable by anyone, and can be declared on the fly if need be.
So, yeah, take Ritual as the cheapest non-smashy magic ability out there. That gives you 2 Focus/4 Potion slots. Then take Refinement as many times as you like for additional 2/4 slots.
Say you're playing in Submerged. This can get you a maximum of 16 Focus Item or 32 Potion Slots. You'll probably want to drop a few of those to keep your final Refresh above 1 (or get some gunslinging stunts). Maybe spend a Focus Item slot or two on your gun (or holster, or glove, or hat or something), to make it into a focus for your bullet-potions. Then spend the remaining Potion Slots on stored Evocations and Thaumaturgy, activated when you fire. Stored maneuvers and stored weapon-damage-increases seem most likely for Evocation, though you could also pull off the Wanted bullet-block thing with a stored block. Thaumaturgy could have some other effects (limited by your Lore + whatever bonuses you have + whatever Aspects you declare, remember), but the only one I can think of off the top of my head would be increasing your Guns rating as you fire.