I have 1300's. What you describe is normal operation. It's supposed to open under recoil. As long as the action is closed tight, until you pull the trigger, it's fine.
Make sure the s/g is unloaded then rack it and close the bolt forward. Without pulling the trigger try to rack the slide back. If it will not come back it's fine. Pull the trigger or push the release and you'll see that it will now open. If it opens without pulling the trigger or pushing the release, then you do have a problem. That's why 1300 Winchester's are called Speed Pump's.
Last edited by dwaawd; January 30, 2013 at 09:20 PM.