Or, if you want to shut it down, the plan works, but since the magic effect has a range, to ball begins expanding before it makes it to the ring, taking the end of the barrel and the ring with it. If the ring is indestructible the character has to go on a search for the ring at the point of impact every time, it i expands within the ring, meaning you have a canon ball that explodes at the end of your gun with shrapnel, or you have to melt the ball down to retrieve the ring downrange.
Or, if you want to shut it down, the plan works, but since the magic effect has a range, to ball begins expanding before it makes it to the ring, taking the end of the barrel and the ring with it. If the ring is indestructible the character has to go on a search for the ring at the point of impact every time, it i expands within the ring, meaning you have a canon ball that explodes at the end of your gun with shrapnel, or you have to melt the ball down to retrieve the ring downrange.