I can understand Greeners concern since the front Receiver ring is split on the bottom.

With the thin chamber wall from the 12 Bore chamber and nothing but a 1/4 inch screw holding the sidewalls together this is not a strong set up.

Dunlap recommends a minimum wall thickness of 2/3 cartridge body diameter for smokeless rifle cartridges.

I don't know what the correct body diameter of the 12 bore rifle cartridge is, but a 12 guage shotgun shell is .883. That equates to a 2.058 diameter barrel cylinder. The body of my 12 guage Greener GP is only 1.310 wide. Okay for a shotgun shells but I don't know about a 12 Bore.