Squib308 mentioned: concur sounds like an ammo challenge. Probably the brass is bulging? have you inspected the winchester brass in advance of vs. following firing? That cutout is wherever the locking lug on the bolt human body locks into. When you appear just forward of that cutout, the extention is https://highimpactrounds.com/product/mossberg-590a1-tactical-12-ga-20in-8rd/