Every DM I've played with: "No guns in my campaign." Every DM I've played with if they allowed guns: "You fire the gun. As the bullet passes through the ring, it instantly expands in size, and mass, to the size of a cannonball. The cannonball instantly loses most of it's momentum, and plops to the ground five feet in front of you. The cannonball did not clear the ring before expanding, so the ring was destroyed and you take 10 damage from the shrapnel."