Two factors there.
Most French SxS users pre war are of the "farmer" type that used the gun often, and probably did not clean much.
The common European rabbit "Oryctolagus cuniculus" was a pest until the 60s, and was a very common and ubiquitous target. Not to mention that it is very good eating. A common foe of the French farmer that was shot at at all times.
Added to that was WWII in which lots of shotguns were buried to avoid Nazi confiscation.
Some did well, the vast majority did not. Of course the outside would be damaged too in those cases.
I believe that a lot of the French shotgun found here came through Germany and were liberated by GIs in 1945.
I don't know if there was anything nefarious in "Poudre T" or the primers used.
Best regards,
WC-
