I like Parashooter's advice on this, I am thinking the round shot rocked a little going through the bore. Make sure the rod you use to push the slug through is flat and square on the end. Using a file and square can accomplish this.
Two other less likely possibilities are some of the grooves in the barrel are filled with copper alloy which would be a little harder than lead or a rough spot in the bore scrapped off some of the lead.

If the .301 measurement was real you would barely be able to see the rifling in the barrel .0005 per side would nearly be invisible. The fact that the .301 measurement is probably so wrong makes your .306 measurement suspect also which is a good thing now.