You’d have to mount it on a wire a bit past the end of the barrel, or custom create a barrel that expands toward the end. Depends on whether dispelling the magic is an instant transformation, or if it “grows” at some rate.

It would have to if the field could help open locks figure it for maybe 6 inches or so, size of a hand + pick.

My physics brain wonders though, in order for a pistol/rifle to fire these we have to assume the mass shrinks as well or else the small amount of powder wouldn’t move it. Once it grows again if the mass grew as well it should just fall to the ground if it didn’t get an extra push…