FX Royale Magazine Not Fitting

"bvan"I had an FX400 that I needed to loosen the top grub screws to allow the barrel to slide ever so slightly forward. I carefully inserted the magazine in place with them loose then tightened the grub screws down. Just be careful not to allow the barrel to rotate when doing this.
bvan, is right in properly fitting a tight mag. 

now is the question--> is the set screw loose before the issue?
 
"bvan"I had an FX400 that I needed to loosen the top grub screws to allow the barrel to slide ever so slightly forward. I carefully inserted the magazine in place with them loose then tightened the grub screws down. Just be careful not to allow the barrel to rotate when doing this.
This would be my first guess as well. If the grub screw(s) holding the barrel in place became loose then the barrel may have worked its way back into a position that won't allow the magazine to fit.