The removeable breech could cause issues. I've seen someone else selling cannon like that where the breech is supposedly removeable only for cleaning.
There is one big problem with that. I used to have a C&R FFL and I follwoed the ATF. A major problem is lack of consistency. The fellow might have a letter from one agent saying this is not a destructive device, NFA, etc.. Another might just pull out the handcuffs.
Personally, I prefer the old style cannon. They just look better.
Steve