Considerations for a 7mm WSM conversion

[BLFields]

I have a 300 WSM and am considering re-barreling to 7mm WSM. However, I want to continue to use Norma brass, so my alternatives are to 1) make it a 7mm/300 WSM chamber and neck down 300 WSM brass, or 2) make it a 7mm/270 WSM and neck up 270 WSM brass.

Every post I've seen which discusses using the shorter chamber speaks of using the 300 WSM case as the parent. Why wouldn't it be better to use the 270 WSM case and simply expand the its neck diameter a mere .007" rather than neck down the 300 WSM case .024"? Is there some reason I've missed that makes the 7mm/270 WSM chamber an inferior choice?

 

[DesertLefty]

Blaine,

My guess would be that the reason is mostly historical. The 300 WSM was available first, so that's what the wildcatters used to make the first 7mm variants.

I agree that the 270 WSM should work fine as a parent case. In fact, probably better. When necking down you often need to turn the necks to get good uniformity. When necking up it's less of a worry. I had great results necking up Lapua 243 cases to use in my 260.

One consideration with the 7mm WSM is that it has a VERY short neck. Winchester did this on purpose so that you can't inadvertently load a 7 WSM round into a 270 WSM chamber.

Thinking "inside the box", the magazine box that is, you could also go with a Remington 7mm SAUM. This will fit a lot better into your Rem. 700, especially when loaded with high BC bullets. Nosler makes high quality brass for the 300 SAUM, which you could neck down to 7mm.

Good luck,

"Lefty",Richard Holmes)

 

[AlanWarner]

Blaine:
I recently worked up a 7/300 using 270 brass. Necking up, chamfers,
and a little neck boring was all I needed to do.,I/m using a .313 neck)
There are a few others doing the same and from reports, the Norma is not holding up well. 180 Bergers @2950 to 3000 are expanding at the rim. - H4831 and 7828SSC
The Win brass is fine. You just have to cull a few. I'm working up 1500 pieces to go with a couple of rifles we are building and found about 25 or 30 pieces,out of 2000) that needed to be tossed.
YMMV
Alan

 

[BLFields]

Richard, no worries on the magazine length since I built the rifle using a Winchester receiver.

Alan, I'm disappointed to hear this about the Norma brass. So let me ask you this: if I were to use Winchester brass, then why bother with the short chamber? Why not go with a standard 7mm WSM chamber? I realize that the neck is short on the 7mm WSM case, but I'm not sure what actual effect that has on the accuracy potential of the cartridge.

I had set the muzzle velocity goal at 2950 fps. For my rifle in its current .30 caliber configuration I have two recipes that yield that or better, one using H4831 and one using VV N560. Is this velocity difficult to achieve in the 7mm caliber? Because this is a practical field rifle, I do not expect to have a 30" barrel on it - 26" is what I'm currently considering - and I have a feeling that the 2950 to 3000 fps velocity range is achieved using longer barrels. Do you have any idea as to what muzzle velocities are being obtained using say 25" or 26" barrels?

 

[AlanWarner]

Blaine:
Used the 270 brass just for the wee bit longer neck.
Only guessing, but @26' you will probably loose 150 feet.
During load testing, I managed 3180. Shot little tight groups @600, but the rims expanded .007" !!!! My goal was 3000 and it works well there for target shooting. I see no advantage over a 6.5 if the Bergers or Sierra 7mm are not going at least 2950. At 3000, the 180's print 2 to 2.5" inside the 6.5 140's.
Alan

 

[rcw3]

Blaine

I have one of the 7mm/300 WSM's built by Alan Warner. The Norma brass is soft and does not hold pressure nearly as well as the Winchester. Frankly, the Winchester brass I have has less run out and seems of better overall consistency and quality than the Norma I obtained. It will be Winchester brass for me with this cartridge from here on out.

The longer neck of the 7mm/270 WSM,vs the straight 7mm WSM) is helpful if you want the option to shoot both the Berger 180's and the Sierra 175's since the back end of the Sierras sit quite a ways up the neck of the case if you have a chamber throated for the Berger 180's. The reamer Alan has is throated as well as it can be in this regard - the Berger 180's sit with the junction of the bearing surface just ahead of the neck and shoulder of the case but no extra, which still leaves things in a good state to be able to shoot the Sierra 175's or the Cauteruccio 176's without running out of neck to hold them.

Robert Whitley