Can anyone tell me what happened here? I was shooting my Mossberg 500 with an aftermarket 18” barrel with a 3” chamber. Firing 2 ¾ inch No. 6 target loads with 1 oz shot was no problem. Then I fired Remington 3 inch magnum 00 buckshot rounds. These were powerful. For four out of 5 buckshot rounds I fired, the locked breach blew open and ejected the spent shell. Everything seems OK with the barrel and all parts. The barrel fits well, fitting tight against the receiver. The barrel screw screws in tight to hold the barrel securely. When a 3” shell is chambered, the slide locks and cannot be pulled back. When the shotgun is turned upside down, I could even see the bolt rising and locking into the barrel recess. Any thoughts on how the action comes open? Is it locking but not securely? Are parts flexing? I’m at a loss for an explanation.