The problem comes when you add a spring recoil system as they have.
If I remember that part is what they have the problem with. You can not use spring loaded devices or a hydro system.
That sends it into a DD zone.
Like I said the recoil system is what changes things.
Then I must be in big trouble (no, not really)...
On various occasions I've been told that Federal law says...
1. Any rifled BP cannon is a DD
2. Any Breech-loading BP cannon is DD
3. Using smokeless in a BP cannon makes it a DD
4. Any BP cannon using 209 primers is a DD
5. Any BP cannon over 2" bore is a DD
6. Any BP cannon that can shoot over one mile is a DD
7. Any BP cannon with a scope attached is a DD
8. Shooting anything other than a round ball in a BP cannon makes it a DD
Never found an individual who could quote chapter & verse in the law that prohibits any of the above though...