Hollowpoint bullets & shotgun pellets of iron instead of lead wouldn't even need spellwork.
They'd still take down a mortal almost as well, but they'd be deadly to the fae. Not even Mab could survive an assault-team so armed (unless she knew ahead of time, and could avoid ever being in their gunsights).
Might not want a hollowpoint. The only thing that affects the Fae that much is the steel. The outside of your bullet is not steel. It's lead, maybe copper. Steel will not take the rifling, and will be inherently less accurate. I don't know a pistol round is big enough for a sabot, so you have the point/penetrator similar to an AP bullet. THAT you make from steel. Actually could be less deadly to humans, as it could well overpenetrate.
Steel shot is a thing. Some areas ban lead shot, and you can get steel shot OTC. Beats down your barrel faster, but you'll get a few battles. Sabot slugs are a thing too.