The Browning Challenger (Belgium made) first model barrel has a removable half circle shaped approx. 3/16th inch insert (which is part of the barrel) which creates a tight fitting once you tighten the barrel screw. This may be missing in your pistol and the best way to check it is to remove the barrel, turn it sideways, and if a hole is showing above the "V" shaped lower part, then that's why it's not tightening up as that part is missing. BT100dc