It isn't a 100 year rule, though one senator was trying to get that passed. It is the pre-1898 law:
https://www.atf.gov/firearms/firear...rms-national-firearms-act-definitions-antique
The part that has always confused me about the law, and which no ATF agent has ever been able to successfully explain to me is the part about ammunition not available through regular commercial channels or produced in the U.S.. By that definition there are almost no pre-1898 fixed-ammo weapons that could be considered antique.
Even stranger is that there are literally hundreds of .22 rifles, 45-70 rifles, 45 Colts and Schofields, .38 S&W, 38-40, 7mm Mauser, 7.65 mm Argentine, and 30-40 Krag guns out there for which ammunition can be found at nearly any gun shop and a lot of box stores.
Forget that second part. I found where the ATF clarified that any weapon made before 1898 is an antique regardless of caliber, but that the ammunition rule applies only to replicas or guns made after that time.
"Thank you for your recent inquiry to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). This is in response to your email in which you inquired about the term Antique firearms.
As defined in 18 U.S.C. § 921(a)(16) the term “antique firearm” means —
A. any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898;
OR
B. any replica of any firearm described in subparagraph (A) if such replica —
i. is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or
ii. uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of
commercial trade; or
iii. any muzzle loading rifle, muzzle loading shotgun, or muzzle loading pistol, which is designed to use black powder, or a
black powder substitute, and which cannot use fixed ammunition. For purposes of this subparagraph, the term ‘antique firearm’ shall
not include any weapon which incorporates a firearm frame or receiver, any firearm which is converted into a muzzle loading weapon, or any muzzle loading weapon, which can be readily converted to fire fixed ammunition by replacing the barrel, bolt, breechblock, or any combination thereof.
The way this definition is to be read is that if the firearm was manufactured
(e.g. made, completed NOT started production) on or before 1898, then it is considered to be an
antique firearm. OR if the firearm is a replica (no matter the year it was manufactured)
and the ammunition used is no longer available through commercial channels, then that firearm is considered to be an antique.
Based on your email, and applying the above definition, if you have a firearm manufactured in 1903 and used ammunition no longer available through commercial channels, then your firearm would have an antique status.
We trust the foregoing has been responsive to your inquiry. Should you have additional questions, please contact your local ATF office. A listing of ATF office phone numbers can be found at
http://www.atf.gov/content/contact-us/local-atf-office
Regards,
Rinell Lawrence
Firearms Industry Programs Branch, ATF"