Directly from ATF.gov
“
When a gas/flare gun is possessed with “anti-personnel” type ammunition, it clearly becomes an
instrument of offensive or defensive combat and is capable of use as a weapon. Since these
gas/flare guns have a bore diameter of greater than one-half inch, fire a projectile by the means
of an explosive, and, when possessed with “anti-personnel” ammunition, are capable of use as
weapons, the combination of the gas/flare gun and “anti-personnel” ammunition is a destructive
device as defined in the GCA and NFA. As a result, registration as a destructive device is
required. Any person possessing a gas/flare gun with which “anti-personnel” ammunition will
be used must register the making of a destructive device prior to the acquisition of any “anti-
personnel” ammunition. In addition, the gas/flare guns are classified as firearms as defined by
the GCA when possessed with “anti-personnel” type ammunition.”
And people are worried about bump stocks… lol