Eliminating over pressure rounds & .38 carbon deposit rings as possible causes I'd suggest rough chamber walls from a graunched up reamer, or lack of finish polishing would be my next guess.
I don't know if you can do this to a club gun, so ask them first.
Take a piece of "Scotchbrite" (not sure if the brand exists there, but its a synthetic green kitchen scouring pad), cut it into a 1/2" wide strip across the pad & cut that in half to get 2 identical oblong pieces.
Take a slightly small cleaning kit slotted jag & bully a corner into the slot. Pull it in tight so it gets frmly wedged. Chuck a length of cleaning rod section into a power drill & screw the jag to that. Wet the pad with your preferred bore cleaner & SLOWLY run the spiralled abrasive back & forth a few times with the drill on LOW speed.
Repeat with the other dry piece.
Take a bore mop that will be a tight fit in the chamber & dampen it with a little brass cleaner. Remove the jag tip & put the mop on. SLOWLY run it in & out about 6~10 times on LOW speed.
Clean the chambers thoroughly.
What you've done is polish & hone smooth any minor marks that "grab" your fired brass. As long as you don't go hog wild you won't change any dimensions, just buff & shine irregularities.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”