[dogtown tom]

kraigwy .....Brings up another point. The FBI runs the Hazardous Device School at Huntsville AL. This is the only civilian LE bomb disposal school in the US.

One of the things they taught us, was how to make a water cannon, which consist of noting but a short piece of pipe with an end cap, there was a hole in the end cap for the fuse/wire, and a 12 gage shell was fired in the pipe. It was a 12 gage shell full of BP and water is poured down the barrel held in with a proteolytic (rubber).

How is that any different then what this guy did?

Seriously?
It's no different than LE having LEGAL access to short barreled rifles, shotguns machine guns, destructive devises, etc. without each officer having to obtain a tax stamp for those items.....its an authorized official use.
Federal law clearly exempts LE & military from certain aspects of the National Firearms Act.....this is why LE can get post '86 machine guns when civilians cannot.