Not only are bullets a physical hazard, but they're an environmental hazard, too. At US Army training facilities around the world, hundreds of thousands of spent shells litter proving grounds. Because there is no efficient way to clean up the shells, they're left where they fall.

But that's a problem. The shells, which contain metal and other chemicals, can rust and pollute soils and groundwater.

The DoD wants to do something about it, though. They're soliciting proposals for biodegradable bullets "loaded with specialized seeds to grow environmentally beneficial plants that eliminate ammunition debris and contaminants."

Such materials best suited for these bullets could include the same biodegradable plastics used to make water bottles or plastic containers. Or it could be some other material altogether.

According to the request for proposal, the US Army Corps of Engineers' Cold Regions Research and Engineering Laboratory has already developed and tested seeds that can be embedded into a biodegradable composite. They've been bioengineered to germinate only after they've been in the ground for several months.

Proposals are being solicited until February 8, after which time, the chosen contractors will produce the biodegradable bullets as part of a three-phase process. In phase one, the contractor will focus on making 40 mm to 120 mm training rounds. "Bullets" might be a bit of an understatement here. Even the smaller 40 mm rounds, which are essentially grenades, are pretty hefty (see image below).