Squib308 stated: concur looks like an ammo problem. perhaps the brass is bulging? Have you ever inspected the winchester brass before vs. right after firing? That cutout is in which the locking lug over the bolt physique locks into. In case you glance just ahead of that cutout, the extention https://highimpactrounds.com/product/mossberg-590a1-spx-12-ga-20in-8rd/