It could be under the barrel and a few inches forward, like on the end of a bayonet.
The ring placement isn’t the problem. Its the rings effect on the cannon ball returning it to normal size.
There’s a reason today we don’t use a few grams of gunpowder to shoot a cannon. The mass of the 5lbs cannon ball is far greater than the energy produced by expanding gasses of a few grams of burned gunpowder. So with a 5lbs cannon ball essentially magically appearing right at the mouth of the barrel (because the ring’s effect), it effectively is a plug plugging up the barrel entirely.
The ring placement isn’t the problem. Its the rings effect on the cannon ball returning it to normal size.
There’s a reason today we don’t use a few grams of gunpowder to shoot a cannon. The mass of the 5lbs cannon ball is far greater than the energy produced by expanding gasses of a few grams of burned gunpowder. So with a 5lbs cannon ball essentially magically appearing right at the mouth of the barrel (because the ring’s effect), it effectively is a plug plugging up the barrel entirely.