As Ridgerunner mentioned, the bolt is the issue. I only know because I was researching the 15-22 and someone showed this to me Sunday.
Below are all of the links from the S&W site to tell you about and what to do. As well a link to order the gauge to check the bolt if you chose that route.
I ordered a new 15-22 anyway, better that it's a known thing and there's already a solution.