Register the 37mm as a destructive device.
You aren't the first person to think of this. Don't bother writing to the ATF. Register the 37mm as a destructive device.
Also, if you want to shoot shotgun shells out of it, make sure it is mounted well. I have seen where the aluminum rails on a AR15 and the mount on a DD'ed flare launcher were broken off from excessive use with a 12 gauge insert.
"No, it's not a silencer. This little doodad is my own invention. I call it a loudener." --Sledge Hammer