[Doug G.]

You don't say if this gun has ever done this before? Have you or anyone else tried to improve the trigger pull? My best guess is that either the firing pin got stuck or the sear bar spring let the sear disengage when you slapped the toggle home. I don't think that it would be the ammo as Winchester primers are not all that soft. The weak mag spring rather than the ammo would probably cause the toggle not to lock open.