As previously noted, unless this problem shows up with other brands of ammo the evidence points to the ammo, not the rifle. Also noted, Winchester ammo has some history of doing this. I have seen this most commonly in old ammo that has been in storage for many years, it seems to sometimes get brittle with age.
This condition can damage the chamber of your rifle and escaped gas could also impact your cheek, or worse, your eye so it's not a condition to take casually. If it was me I would not use any more from this box or any other box with the same lot number. Be prepared to ship the fired cases including the damaged one and all unfired rounds, in the original carton, to the manufacturer, along with a complete explanation, rifle details and good quality photos. Do not attempt to open, section, cut, split or otherwise alter the faulty case, they want to do that themselves and need to start with the unaltered case, just the way it came out of the gun. Contact the manufacturer with your complaint and be polite, explain your concerns and offer your assistance. They will do their best to cover their ass these days so be prepared to be ignored or at least stonewalled, they won't easily accept responsibility. If you can't get a response or if they blame you or your rifle your best bet then would be to persue it through a Winchester dealer (store) or a reputable gunsmith. Good luck.
