Scope eye. If my figures are accurate......
The lapua case is shorter than the RUM by 0.126"
The lapua is only 0.019" wider at the case before the shoulder and only 0.006" bigger at the base.
I don't think cutting 3" off the lapua barrel would be necessary. With a little more calculations I think you can shorten it just enough to clean up the lapua chamber with a 338 lapua reamer. I don't see these numbers being "a lot" If I'm wrong please explain.
Thank you