Yeah, when I shortened my inherited '97 Cyl bore to get rid of the polychoke I specifically had the 'smith shorten it to 18 1/2" just in case I ran into an ATF agent with a "short stick."
ANY smoothbore, save muzzleloaders (and I wouldn't mess with them there either, they are probably as unintelligent with them as well, just like "suppressed" air rifles.....
), it's no LESS than 18" barrel AND no LESS than 26" overall....