257-300 wsm

kevin0270

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Joined
Sep 1, 2012
Messages
57
Does anyone have any reloading info on this caliber? I have searched the internet and have not found anything? Thanks
 

kevin0270

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Joined
Sep 1, 2012
Messages
57
I would look more towards the .257 SS and .257 SST.

You can find info right here on this site...

https://www.longrangehunting.com/threads/quarter-bore-sst.204569/
I would look more towards the .257 SS and .257 SST.

You can find info right here on this site...

https://www.longrangehunting.com/threads/quarter-bore-sst.204569/
Thanks but A friend of mine has a rifle chambered for this round and wants me to reload for him.I have the reloading dies but no info on bullets and powder charge,Thanks
 

257Tony

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Joined
Apr 8, 2013
Messages
797
Location
Utah
Does anyone have any reloading info on this caliber? I have searched the internet and have not found anything? Thanks
No experience with it, but looking at case capacity vs 257 wby, I would bet 7828 class of powders would be a good place to start.
 

MagnumManiac

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Joined
Feb 25, 2008
Messages
2,285
I, and a good buddy, built one the same year the 300WSM came out. Both on Win Model 70 Classic Stainless LA actions.

It was a nightmare to load for, pressure would track nicely from start load to mid-range then it would spike unpredictably, or we would get under-ignition with dented and sooted case bodies all the way to the case head. Not until we passed or matched the max loads for the 257 Bee would the ignition problems stop.
At first we thought we were using powders too slow for the case capacity/bore ratio, but it was a case fill issue.
The ONLY powders to behave without issue were RE25 and N550/N540/N560 & N170.
Not a single powder like H1000, H4831, Retumbo or
IMR7828 behaved as we expected. Unless you have a near full case of powder and some compression by the bullet, you WILL get under-ignition and pressure spikes.
Bullets lighter than 100gr were the worst offenders and we found decent accuracy with 110gr Accubonds and 115gr Partitions with max loads of RE25.

My only advice is to start with 257 Weatherby data and SLOWLY work up from the start loads. Any under-ignition signs; dents in the case body, sooting or misfires, INCREASE the load to 5% below MAX and work up.
Traditional powders for the WSM cases failed to work for us......YMMV though.

I can’t give you data, it went long ago when I re-barreled to 270 Weatherby.....much better cartridge.

Cheers.
:)
 

MagnumManiac

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Joined
Feb 25, 2008
Messages
2,285
It’s also called the 257 Pronghorn, that may help you find more info.
Yes, my dies and reamer were called ‘25 Pronghorn’, although both RCBS & Redding have listings under ‘25-300WSM’.

There is data out there under the name of 25 Pronghorn.
‘24hr campfire’ may have some data, also try ‘accurate reloader’ forum.

Cheers.
:)
 

CjC73

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Joined
Jan 21, 2018
Messages
1,175
Location
SE Mich
I just recently bought one. The guy I bought it from gave me his load data. I can provide that...I've yet to try it.

I'm currently on vacation and can send it to you once I get back. PM me if you want.
 

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