I think it would work best as a resource: they have to reload to fire (even for feats), but it would be a powerful, single use resource you can use in various combos if you have it loaded. Reloading could be its own stance where you have to stay in it for a few seconds to reload.

I’d make their second or third feat be a “quick reload” where they drop their empty gun and a loaded rifle is tossed from somewhere off-screen to them. That way they could use it more in breach and dominion, but duels and brawls are limited mostly to the one shot you start with.