You must have a different RCBS funnel than I have. I just bought it because the Old Lee funnel I have been using for years, didn't fit the 300 RUM brass.
Take a couple of pieces of brass to the local Gun Store. They should have one that will fit.
I have a powder funnel from midway usa that is caliber specific with two universal adapters also a long drop tube. They work
great on 22br,.223rem,6mm/06ack.,270wby,7mmrem mag,300rum,and 338/300rum. I highly reccomend this funnel kit, works great.