What is the Lore for your character? The main issue I see is that to produce an equivalent effect to the Warden Sword, (A counterspell at Fantastic (+6) strength, plus maybe a weapon value increase) you'd need a fairly high lore to create.
Otherwise yeah sure, I'd certainly allow magic bullets to be created. Loading for that turn as a supplemental action, and then using straight away (though the Fast Reload stunt, or using a supplemental action on the previous exchange would allow you to shoot without a -1 penalty to accuracy).
Another possibility, and one that might be more reasonable if you don't have crazy lore skills, is more general magic bullets. Force bullets for extra damage, sunshine bullets for some of those pesky vamp critters, silver bullets for werewolves and holy bullets for any demonic types you run across. Devote two enchanted item slots to them and have an aspect like 'Bullet with your name on it' or whatever and I'd probably allow you to either roll for, or to automatically have, the right bullet for a situation 3 times a day.
Wouldn't mordite bullets dissolve the gun? And the gun holster? And your leg?