I've done this in the shop one other projects to fit tubing.

Take a heavy block of steel long enough to roll a 10ga hull across and place a bar of steel wide enough to cover the whole case or enough to cover the amount you want to size. Leave the rim hang over the edge and roll the case back and forth between the bar and block increasing pressure until you sized it enough, apply pressure in small increments so you don't make it oblong.

After the first shot the rest are just noise.