Re: 338 rum 250 matchkings
Good deal on having a summer to enjoy.
Here's the hard way: Take a dremel tool w/cutter blade and cut the nect on a case from mouth to shoulder creating 3 equidistant slots or, very slightly neck size a case, only enough to hold a bullet when pushed in w/fingers.
With the bullet barely seated insert the cartridge into the chamber. The bullet will then be seated by the rifling. If there is sufficient neck tension the cartridge when removed will at "a" maximum overall length.
You could use that for a starting point but the RUM may well have a very long throat.
Here's the easy way: ensure the cartridge fits and feeds from the magazine and go from there. That usually sets the longest practical length.
Good luck and good shootin'
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!