Yea the sandpapering did nothing to correct the issue anyways. The slide lock is still holding fine in place as when it was new so that's not an issue.
The part about me fighting against the magazine's spring tension makes the most sense here. I'll have to load it up and let it sit a while and see. Appreciate your help guys.
”Gun control is like trying to reduce drunk driving by making it tougher for sober people to own cars.”