Apparently ATF
Rulings say if a taser projectile is launched by powder it is a firearm but if it is launched by compressed gas it is not a firearm.
Taser International reports n MS Word format are a useful starting point on the topic, see
State Statue Summary and
Memorandum of Law.
A number of deaths associated with police using tasers have been reported.
Should ATF classify all tasers as firearms or retain the present ruling that some are firearms and others are not firearms?
EDIT TO ADD THE FOLLOWING FROM THE "Memorandum of Law" LINK ABOVE
QUOTE
Legal Restrictions on Use of TASER Conducted Energy Weapons
A TASER conducted energy weapon is not classified as firearm by the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives since it uses compressed nitrogen gas as the propellant. Therefore, firearms-related regulations do not apply to the sale and distribution of the TASER conducted energy weapon within the United States. A distinction needs to be made with the old Tasertron conducted energy weapon that used black powder for the propellant and was classified as a firearm under U.S. Federal and state regulations. The Tasertron unit is no longer being manufactured.
Some states have enacted regulations restricting the sale and use of inexpensive, hand-held shock devices and these regulations also apply to TASER conducted energy weapons. In many cases, the law enforcement and corrections market is subject to different regulations than the consumer market. Based on a review of current regulations, the following states regulate the sale and use of the TASER conducted energy weapon:
UNQUOTE