Interesting proposal. Hadn't thought about spent ammo, but it's a huge problem. The issue there is safe transport and handling, but that's prolly a minor issue grand scheme. More interesting is the encapsulation tech needed to 1) block the relevant radiation effectively enough for it not to be a proximity hazard, 2) to be stable enough for long enough to see the dangerous half-life out, and 3) to encapsulate the lead as well as the radioactives. Not sure if glass encapsulation does that...again, there's science to be done here.