Ok. This is a tough one. What I would do is pull the scope. Remove the rear screw on the front base and see if the base moves any when you tug on it. Since your gun isn't sighted in it won't make any difference. If it's loose then remove a little off the bottom. I know you say it's tight but I've known veteran shooters who havehad loose bases. It won't take much to make sure. Scopes are always suspect so you may want to put the sights back on and see how it shoots with them if it's not the base. Other than that a new breech plug would be a good investment. Sometimes they just don't last.

If none of this works send it back to Knight.