Redding Comp seater runout?

E

elmerdeer

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I have a RCBS Rockchucker supreme press and my runout on my 7mag is very good between .0005-002" I use lee collet necksizer and a Redding comp seater with nosler brass and 160grs accubonds. So now my problem, I also have the exact same setup for my 280AI using the same dies and brass, and the bullets are 140 accubonds, so why do I get runout of 0035"-009" out of my setup for the 280 AI? I have tried using an O-ring under the seater and it got a little better around 003" to 005" but that is not what I like. I am also waiting for my gunsmith to ream my Forster comp seater with the reamer used for my chamber and then I can compare if it gets better. But aside from that why is the 7mag get much less runout than my 280AI?
Thanks
Elmer
 
If some of the cases are coming out of the sizing die at .0035" you are beat before you screw in the seating die. Assuming that the cases are straight coming out of the chamber, what is the neck diameter of the fired cases compared to that of a loaded cartridge? If the neck die has to reduce the neck too much it will often get crooked. 2 or 3 thousandths clearance is a good fitting custom chamber, many factory chambers run up to .016-.017 neck clearance. Much over 4 or 5 thou I've found that I may as well put the collet, neck and bushing dies away and just go back to the full-length die.
 
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