Thanks for all the great info. It looks like tires will do the job. One of my hesitations was what to do with 20 tires if they didn't work, as we have to pay $5 or $10 each to get rid of them. The answer is that they "will catch fire from unburnt powder!"
Will drill holes in the bottom for drainage, and bolt or cable them together. They will need to be on some sort of mobile gantry, as I shoot from a bench for load development and benchrest practice, but shoot prone or off a backpack for hunting practice.
Next issue is how to trap the lead. My range will probably encroach on federal property, at least beyond 300 yds. I need to be able to pick up my lead and leave if so ordered, and not get involved in remediating lead. . So far, my idea is to put 2ftX2ftX4ft long treated plywood boxes on legs. Boxes would be filled with sand, and covered with metal roofing to keep them dry. Couple of baffles in front of the box to keep sand in box as it gets shot up. What say ye?
