OK, explain this to me, if the ATF requires 26", how can these shorter lengths be legal?
Some state allow you to get special licenses for things like short barreled rifles, silencers, and full autos.
Federal, and most state, law has *two* separate criteria, both of which must be satisfied if a shotgun is not to be deemed a short-barreled shotgun subject to the National Firearms Act rules (which are pretty much, but not exactly, the same as those for machine guns). One is that the barrel or barrels be greater than 18" long, *and* that the overall length be less than 26". With a pump or semiauto, it's pretty hard to get under the 26" overall length if the barrel is 18" or more long. Not so for a single or double barrel break action shotgun. The only National Firearms Act case ever to be adjudicated by the Supreme Court involved a break action shotgun, which violated one and likely both of those criteria. (US vs Miller).
It's really too bad Miller didn't have a Thompson or a BAR instead of cheap Stevens break action shotgun. Things might have been different, since the Court would likely not have ruled that the lower court should not have taken judicial notice that one of those was a weapon suitable for militia purposes.