marklbucla - as others have suggested, in removing the "dimples' in the magazine tube, you eliminated a component of the system that the Express guns use to keep the mag cap in place.
The sling adaptor will not solve this problem.
I've been looking for a cheap fix for the "dremmeled Express" problem and tried the detent sling adaptor myself.
Unfortunately, the detent ball in the sling plate is not spring loaded. It just floats. So, you can't get it to line up with a dimple as you tighten the mag cap (it just backs up against the unyielding barrel ring). I tried sandwiching some foam between the detent ball and the barrel ring to give it some "spring," but this did not work either. If you have a solid barrel ring, a detent-ball sling plate/dimpled mag cap will not fix the problem.
Solutions seem to be (others, feel free to add to this list):
1. Buy wingmaster/police barrel with detent in barrel ring (and appropriate mag cap)
2. Have detent put in Express barrel (again, with appropriate mag cap)
3. Periodically hand tighten/tape threads
4. Use a magazine extension and clamp