The Final word on that of course comes from ATF.
That being said this is what we accept on this forum.
1. Any firearm not designed or redesigned for using rim fire or conventional center fire ignition with fixed ammunition
and
2. Manufactured in or before 1898...or replica thereof, whether actually manufactured before or after the year 1898)
or
3. Any firearm using fixed ammunition manufactured in or before 1898, for which ammunition is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade.
We came up with this board rule after several reports in the media of people with bowling ball mortars made from welding tanks and set up with Stokes mortar mounting gear being arrested and charged by ATF.
Seacoast can build, shoot and discuss their because the Mortar theyare making replicates a mortar made before 1899.
If the guy had built his gun to look like a French 75, no problem here
You can read our discussion on th is board rule at:
http://www.gboreloaded.com/forums/index.php/topic,105055.0.html