I've just commissioned a new ball milling round pit for the safer milling of black powder. It was an unused barbecue pit, designed to cook the heads of deer, pigs, and other favourite large game animals, but never was used for such. The sides and bottom extend 2 feet below grade and are lined with fire brick and refractory cement. The lid is aluminium and provides for protection from rain. Tyres are used for vertical shielding against airborne projectiles such as the lead balls and jar shrapnel. It's located 100 feet away from my assembly shed, and even farther from dwellings. A larger rectangular pit is to its right of this little round pit, and I got tired of having to shovel out the ashes just to use it for milling, and now have a dedicated pit just for pyro.