During my regularly schedule Tuesday Skeet night, my 500s trigger housing pin (#27 in the schematic) kept sliding out, once it came completely out of the receiver. Disconcerting, to say the least and embarassing. Interrupts the smooth rhythm my flight tries to maintain. Have had this gun 30 years and shot it alot. Replaced several sets of extractor springs, magazine springs, etc but nothing like this. Any ideas about what is happening and why?