Neck up or down?

H

hogwild

Guest
I'm having a 338 O'brian (338edge improved) built and was wondering which is a better way to go, neck up 300 RUM to 338 caliber or neck down 375 RUM to 338 caliber? The GS recommended necking down but said I could do either one. Once the cases are necked they will be fire formed to the 338O'brian.


Pro's and Con's of necking up or down?
 
Neck down whenever possible for best results. One added bonus with this method is that you get to set your own headspace since I'll bet dollars to donuts your new chamber will not headspace to a crush fit on necked up brass.


If this doesn't make sense then just do it anyway since there's no advantage of any sort to necking up :)


al
 
Neck down. When you neck a cartridge up it always stretches at the thinnest point making an eccentric neck even worse.
 
Very good points have been brought up.....

There is one very important thing to remember, when you neck down, you brass gets thicker, especially at the shoulder neck junction where you set the headspace.

It is not uncommon for necks to get .002 thicker or more when necking down cases of this size.

Thicker brass may be good or bad or neither depending on how you elect to deal with it, and you will not know until you start working with the brass and dies that you have.

I have done a lot of both necking up and necking down.

The use of a long tapered expander along with good lube discourages a lot of the thin spots that can happen on necking up, and opening up the brass in stages is always easier on the brass.

When I am necking down brass, I almost always end up neck turning the brass, and a person always has to be aware of headspace issues, firm contact with the shoulder neck junction is always preferable.

Things seem to get very wierd as a guy gets real thick brass. The Brass will work harden, then you get a SUPER grip on the bullet causing pressure spikes. Combine that with the fact that brass may vary in Rockwell hardness as much as 13% in a lot# of brass and you can get variations in bullet grip that will drive an accuracy nut crazy. Annealing after the 3rd firing or more may be something that you want to consider, you will know by how consistant your bullet seating is going.
 
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