I could see that it is chambered for a belted cartridge case.
When I chamber double rifles, I measure the rim and/or belt thickness of maybe 20 new cartridges and I also refer to the cartridge original design specs and then I chamber the cartridge for only a couple of thousands of an inch clearance between the back of the cartridge and the barrel flats. Then after shooting if I see that another thousand or so of clearance is needed I increase the depth of the cartridge rim into the chamber. This is a very critical clearance on a .375 belted magnum as the pressure of this cartridge is significant (max 62,000psi) versus the .375 flanged cartridge of 47,000psi.
I suggest to you that you measure the depth of the belt cut from the face of the barrel flat in the right barrel and do the same for the left barrel and compare the two measurements. Check to see how close the chamber cut is to the standard of the .375 belted mag which I believe is about 0.220 inch thickness. You can look up the standard for the brass on the web.
I just went back and looked at your photos again and I can see just barely an imprint of the cartridge rim of the left barrel against the action flat. Therefore my previous thoughts may be wrong about why there may be some gas escaping from that barrel---if there is any excess escaping.
Stephen Howell
Last edited by bushveld; 09/23/23 05:13 PM.
