Squib308 mentioned: agree seems like an ammo challenge. Probably the brass is bulging? have you inspected the winchester brass prior to vs. after firing? That cutout is wherever the locking lug to the bolt body locks into. Should you glance just forward of that cutout, the extention is a little https://highimpactrounds.com/product/mossberg-590a1-spx-12-ga-20in-8rd/