I have a 300RUM, and have had a few. And I like them. They are finicky. They will have large velocity spreads upwards of 50-70fps. This is pretty well understandable with a overbore cartridge like the 300RUM that uses upwards of 101grs of powder. Also, there is some freebore in the chambers. It helps get to those velocity numbers safely.

Inheritly accurate calibers generally don't have these attibutes. So if its too hard for your gunsmith to understand, you need to find a new gunsmith. I like the RUM's but also understand how it can be inaccurate. I have a few loads that are .5MOA. But it takes twice the amount of work for me to hone in those numbers. than it would a WSM or 308win.