With the problem some of us having in finding the correct size projectiles for our bores. I thought this idea might work. Not having a mill to cut a slot in steel. I made one out of wood. If you think it will work I'll turn one out of metal and slot it with a hack saw.
The thought was to turn a cylinder to the size of the bore minus the thickness of a piece of sheetmetal and the allowance for windage. Then mill a slot the leingth of the cylinder about 1/4 to 1/3rd the depth of the diam. of the stock. Cut your sheetmetal to size plus a 1/4" on the sloted ends and bent these 1/4" 90 deg. Then bend the metal around the cylinder clamping it with an automotive hose clamp. Having done that the seam would be soldered. Then one overhanging end would be hammered flat for a bottom. Then filled with concrete. I think that filling it with lead or zink without a form might distort the sheetmetal.
Well have at it and see what you think.
Div Arty
