LOL
you were right...here it is. looks like a barrel extension some sort. cheap too. under 300$
http://www.metrogun.com/
odd.it says super quiet, like a car door closing. but I can't find any NFA info. just says 50state legal by atf. says like you stated, usd by city's/airports etc. for pest control
Question: Is Metro Gun legal?
Answer: Yes, the US Government agency, USDA, uses the Metro Gun System for nuisance pest-control as do several other agencies. This gun system has been cleared by the Bureau of Alcohol, Tobacco and Firearms, and is not considered to be a silencer.
Question: How quiet is the system?
Answer: 72 Db, or about as loud as a car door closing, or two people talking at a range of about 150 feet. (Just above the level of a silencer.)
edit: after reading, looks like its a total system to get that quiet, combining barrel/choke extension and most importantly....their own subsonic quiet ammo. could just be a giimick.......32" BARREL!!!!
the videos are very vague, but it does seem quiet, I guess you could handload your ammunition or buy subsonic and get similar results if you just bought their barrel system. I would have to find someone firsthand though before buying