I wanted to thank all of you for your suggestions on a muzzle brake for my 300 RUM. I settled on a 5 port stainless two piece super baby beast from muzzle brakes and more. This thing is a work of art! For those of you concerned with Nathan's(owner) method of communication, let we reassure you he is trustworthy.
We exchanged emails and he sent me the unit timely with a taper that matches the barrel. .650. This threading was unusual in that a previous smith threaded the barrel 13mm X .75 pitch.
Fit and finish on this unit are superlative and I will use him again.