Rum uses a standard long action (magnum bolt face an extractor) it would be the same length as if you had a 300 win mag, 7mm Remington mag, 264 win mag, 30-06, .270 etc. Just like Jeep kid said the COAL of a RUM is too long for these actions. I noticed it when I got my 300 ultra but it isn't that much of a problem if any at all. Just an announce more than anything.
https://support.remington.com/Genera...hort_action%3F

Honestly if you are building one just look for a used Savage action or savage rifle and buy a barrel. The route I went was I bought a Remington SPS 300 ultra and switched the stock out adn the trigger, that trigger is crap. The SPS shoot great at least from the ones I have had. I have not got to shoot the 300 yet do to I am still waiting on my Vortex scope.