It's not the length that makes it qualify in this case. It's the fact that it's considered a disguised device. It doesn't look like a shotgun.Is it an AOW? Presumably the barrel would be over 18" if it is in a cane. And it would be over 26" overall (at least my canes are).
The type of weapon you want to make is regulated under the 1934 National Firearms Act. A cane gun is an "Any Other Weapon," and requires an application to BATFE to manufacture or purchase. You apply and pay your fee, BATFE eventually processes your application, you get a tax stamp that authorizes you to manufacture or purchase the device you told them about.What's an AOW?