Making 257 wby brass from 7mm wby brass

SofaKing

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I’ve searched and can’t find the post I read about this. So can you use 7mm weatherby mag brass and resize it to 257 weatherby mag? I’ve never tried doing anything like this before hence the question.
I have a rcbs 257 wby mag die set. Would that be sufficient to reform 7mm wby mag brass? Any issues I may run into?

Thanks
 

Dmagna

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You can do it, just make sure the necks are not too thick. The 257 270 and 7mm are pretty much the same. Just can’t start load development until you fireform, just do a couple to make sure you don’t have to turn necks. Because then It gets way more complicated
 
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trigger103

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Yes it can be done. My brass is always just a little short. (Haha no comments on that). but it does work. Just lower your sizing die a little on each pass and rotate the brass. Trial and error just go for it.
 

Dean2

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Easier way to size it down, take the stem out of your 257 seating die. Run brass through it first, then the sizing die. Your necks should not get appreciably thicker, but measure to be sure.

This one, no neck turning needed.

I have found it a whole bunch easier to find 378 or 460 brass and just neck it down. Seems to be a lot more available than 338-378.

460 brass, 1st step, 378 seating die, 378 FL die, 338-378 seating die part way, 338-378 seating die rest of the way, 338 FL die.


QKpMtTTl.jpg
 

Hoss50

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I made them from 7mm Rem Mag and 264 win mag brass in my last barrel. This barrel has Norma brass, but I think I might switch to 270 weatherby brass fro. Petersons and size it.

I have size 270 weatherby brass down to 257 with no issues also.

The nice part about 264 win mag or 7mm rem mag is they end up short so you don't need to trim cases much as the grow. They have a slightly smaller case capacity from what I read though. They shot fine til the primer pockets went or the necks cracked.
 

DesertBoy

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Can you just size the 7mm brass with the techniques stated above, and load them with a healthy powder charge? Or do they have to be fire formed first? Sorry for the ignorance, I am new to fire forming.
 

Dmagna

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Can you just size the 7mm brass with the techniques stated above, and load them with a healthy powder charge? Or do they have to be fire formed first? Sorry for the ignorance, I am new to fire forming.
It’s the same case. The 257, 270, and 7mm all share the same case. Just need to run it through a die and shoot it, everything should really be foreformed before you start load development. I would just resize and use the first firing to break in the barrel.
 

Dean2

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I have often used newly formed brass to do the first round of load development. Testing distance off the lands with a mid load works just as well with newly formed brass as fire formed. After the first firing I then work up the powder charge to find max velocity that holds a good group. Done that way you aren't wasting components and you gather useful data with every firing.
 

M1A1ABRAMS

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