this is happening with steel lipped BC mags,BX 25 mags too. so far it has not done it with factory 10 rounder. one fellas guess is the recoil spring has weakened over time,and the bolt is slamming rearward a tad too hard,causing the tab to flip up and hold the bolt open. the carbine is about 18 years old,i'm second owner. a friend is going to bring me a new take-off spring assy he has,since it will be art the speed steel match I SURE HOPE IT FIXES IT. Otherwise it's gonna be a loooong match. I'm open to thoughts on this one. I will clean up the trigger group this morning and go run some more mag tests .