I doubt it could grow enough on first sizing to reach lands. You shot all 50 rounds and only one looks like that?
With that damage how hard was it to extract? I am surprised if it was easy to remove and if it was then the damage happened after removal.
"Anyone taking up handloading necessarily plays with unknown factors and takes chances. But so does anyone who drives a car,goes to a cocktail party,eats in a restaurant,or gets married."
Jack O'Connor 1963