Help me understand this 300WM load

[MojaveJim]

That's why I was thinking new rough bore getting some copper the first few rounds or different copper alloys not getting along with each other.
Parallax could do some but not 4".

 

[Duh]

I’m going to take it out again and check it at distance. Hate to just chalk it up to a fluke but I can’t figure out a solid theory.