Agent called me back to clarify something we discussed (as their office was closing no less)...I wasn't at a pooter. He emphasized that if it is not regulated by ATF that there would be no need to register it. But that even if it did you would only have to pay a $5 registration fee. He said if the agents were really that concerned about it, they would have been at your house within an hour or two.
These things rarely surface and ATF isn't really concerned about them too much...even those that have to be registered. "not too many gangbangers or terrorists interested in antique tear gas launchers"
Couple of other interesting things he mentioned:
Perfectly legal to own tear gas.
If it were rifled so that it could conceively propel a grenade it would be illegal.
You can even register a pipe bomb legally if you're so inclined.
He did a follow up call right as he was leaving his office but the office in DC/WVa was already closed (an hour ahead of us). He'll get back to me tomorrow with a definitive answer. If they show up before you get an answer, your recollection with grandpa was that it fired a low pressure 12 ga blank that has long since been NLA. It was obsolete long before he retired from the force. He used it as a paperweight.
Hope this helps you sleep tonite.
