RE: 300 RUM total length
A 300RUM has so much freebore, you can probably go up to 3.77" and not touch the rifling. So what I do is load as far as I can and still fit the shell in the gun. Which is 3.63". Believe it or not, your velocity/pressure will probably go down some by going from the standard 3.57" (factory) to 3.63".