Perhaps I've misunderstood pocketshaver, do you mean reforming .303 brass to form .410 shot shells?

Otherwise, there's a massive difference in pressure between a .303 ball round and a .410 shot shell ... Even taking into account a .303 round bouncing down a .410 barrel with all the accuracy of throwing a sausage down the high street, as gas pushes past the bullet, the initial pressure would be 'way above' the designed, proofed and safe pressure in a paper thin .410 barrel.

Looks like a clear case of Darwinian evolution theory in action to me, great way to loose your sight and have two hooks for hands!