Hi,
I have a factory barreled Weatherby Vangaurd in 257 WM.
My brass neck thickness is measuring in .283" (with bullets seated). I am using 7mm Rem Mag W-W brass re-sized to 257 WM.
Some manuals say the maximum outside neck demension is .281", while another is showing .283", and yet another says .285".
They ammo chambers properly and I am not seeing cratered or expansion at the case heads.
Should I turn the necks down to .281"?
Thanks

Make a cast of your chamber neck first.
There nothing like the actual measurement to refute the reference books.
if you have a 7mm bullet = .284
See if it will drop into the chamber neck base first. That will tell you it is at least .284.
What you need is a drawing of the minimum chamber not the maximum cartridge. Even that does not tell you what your chamber is really cut to.
You could also wrap a .277 bullet with Scotch tape up to .285 or .286 and see if it fits the neck of the chamber.

One last trick would be to seat a .264 bullet backwards and very deep (flush) in your cases.
It will expand the neck .007. If it does not chamber turn it down until it does and you will know what your rifle chamber measures.

Buy a box of .257 Wby factory ammo. Fire a round, and measure the expanded case neck diameter. It will be about .005" below the neck diameter of your chamber.

If it is close to .285" (your minimum safe chamber neck diameter with .283" loaded necks), a chamber cast is not a bad idea. But a lot of factory chambers are on the large side, so I would not be surprised if your chamber was (quite a bit) larger than minimum sizes specified.

To be safe, your chamber neck diameter must be at least .002" larger than your loaded neck diameter. For a hunting rifle, .003" or .004" is not too big either.

- mike

Based upon my range results, I think this idea of resizing 7mm Rem Mag brass to 257 Wby is a bad one. The cases are to short, they are mic'ing in at 2.480" on average. Perhaps it would have been better to start with 300 Win Mag brass.

The loaded ammo had a diameter of .283" with a bullet seated, once rounds were fired, the brass diameter is unchanged and I am unable to slip a 25 cal bullet into the cases.

The interesting thing is that I am not seeing signs of excessive pressure, no flattened or cratered primers. Primer packets are still tight and the case heads are still in tolerance.

Bottom line, I went to Sportsman Warehouse and bought two boxes of factory brass and I am going to sit the re-sized 7mm RM brass aside for the time being.

Thanks to those who kind enough to share their thought and offer good advise.

Don

My custom 257W has a chamber neck diameter of .288.

The new Norma 257W brass measured about .282, and .284 when loaded, giving a clearance of about .004.

However, I found that the Norma brass was consistent, but the reputation for being soft was also correct.

The velocities I achieved in my 257W using Norma 257W brass were only insignificantly greater (10 - 30fps) than those I am comfortably achieving in my 25/06AI.

I thought about trying some Remington 7mm RM brass, but like you after necking down, found that the length was 2.480 - 2.485. I asked my gunsmith whether this presented a problem, and said he wouldn't do it on a long term basis.

So I bought 50 Remington 300 WM brass and necked them down to 257W. The loaded rounds ended up at about .2865, so I neck turned the cases to give about .003 - .004 clearance, which I believe is more appropriate clearance for a hunting rifle.

I found that the capacity of the necked down 300WM brass was about 2.5 grains less (obviously thicker) than the Norma brass, and at maximumum pressure used 1.0 grain less powder and produced about 75 - 100 fps more velocity.

I recently bought some Winchester 300 WM brass (gunshop didnt have REM) and will neck it down and compare the results. Hope that the above is of some help. Brian.

I tried R-P 7mm Weatherby brass necked down in two .257 Weatherbys, they gave thicker necks than Weatherby brass for a "tighter" neck clearance. The loaded neck diameter with the R-P cases was 0.286" - for the Weatherby it was 0.284". The fired neck diameter in my rifles is 0.2875" so no problem with too tight of a neck.


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