Not all steel barreled guns were Nitro proofed...especially when there are no barrel flats...often when the barrels are round where they mate to the receiver, the gun was not nitro proofed, even though fluid steel barrels are present...the only Swiss maker that I can think of is Husqvarna (I've not seen a Carl Gustov SxS)...I don't think any of the Husky's with Lefacheaux lock-ups were nitro proofed...the best thing is to post a pic of the proof marks, and these guys will amaze you with their understanding of proofs and acceptable pressures...From those marks, I and others, will probably be able to tell you the name of the final assembler of your particular gun, if in fact it's a Husky
