Re: Which dies are you using for 300 RUM?
I was told the rockchucker II was not tall enough for the Ultra. I load mine on the Lee Classic Cast single stage. If you have a RCII stick a case in there and see if you have enough room to seat a bullet.
I don't full length size mine so i dont have a suggestion there. I am having a Lee Collet die made to neck size, and will use a Redding body die when necessary.
If I was you, I would be tempted to start with a Redding full length sizer, and try to get the expander button stem centered in the die. There are several threads her on it, and people mentioned being able to get minimal neck runout with it.
I believe redding sells a die set with the standard full length sizer and the competition seating die. Thats what I would get.
The problem I ran into with my 1st press, is that after switching to the comp seating die I ran out of room on my press. The comp dies have a seating stem that protudes down below the die body.