Extraction vs case taper.

[4xforfun]

I have a 6.5 WSSM , or 260 WSSM....what ever you want to call it. I was reading another thread on another fourm and , at least with BIG cases, it was sugjested that anything less that .010 - .012 inches of taper per inch can cause bolt lift and extraction"issues".

I have had my 6.5 since 2008. I use Win 25 WSSM brass, and based on the fact that it extracts hard, I figured I beat the snot out of the brass. But, the PP's are still tight as new after 6 - 7 firings. One of the things that I noticed is that it didn't matter weather I used 43.5 G H4831 or 48 grains behind A Berger 140, the ejection issues were the same. It just didn't matter. Re-chambering a fired case was just about impossable unless I re-sized it first.

I measured some fired brass, and best I could tell I was right around .0065" taper per inch. I measured a bit down from the shoulder and a bit up from the grove where the case swell is biggest.

I also measured a few Dasher brass. It looks like (the way I measured ...down a bit from the shoulder and up a bit from the grove) I have about .0085 of taper , but that is only in about 3/4 of an inch. I would rough out the "actual" taper at around .010 per inch...give or take.

Any thought on this? I love shooting my 6.5, but keep thinking I am doing something wrong because of the extraction issues, so i don't shoot it as much.

Oh ya.....this is on my BAT M 1000 yard comp gun. I have ZERO issues when I shoot it with any of my 300 WBY or WSM tubes.

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[Nvreloader]

4x

Go on line and look up,

"How to figure tapers",there is a set of formulas that show how to figure the tapers of a case,
it would be very helpful for you figuring on what you want to do etc.

I use these formulas for figuring out my wild cat cases etc.

Tia,
Don

 

[conor m]

this a problem i came across with win brass , simple fix change to different brass manufacturer , its down to the thickness of the extraction rim on the case . i went to norma brass ,helped to remedy the problem . norma brass is 4 to 5 thou thicker .
all the best and hope this helps conor m

 

[savagedasher]

6.5 WSSM is great shooter. The brass is a big problem. The brass is too thick With a case that short the brass doesn't expand in the chamber . So you end up without any spring back on the brass.
Here is something that will help. Have your bolt face and chamber polished. The case doesn't expand enough to be held in the chamber. The case only springs forward. You will find the case has the same diameter but the head space I gone. Annealing helps also. Larry

 

[Laurie]

On the face of it, a heavily tapered design should extract more easily than one with less, but that doesn't seem to work in practice. One of the claims for P O Ackley's 'improved' designs is easy extraction despite the taper being significantly reduced on most designs during fireforming compared to the factory 'unimproved' shape.

This appears to be a result of the less tapered case gripping the chamber walls tighter at peak pressure and also early in the combustion cycle as pressures rise, thereby reducing backwards movement / case-body stretch in the chamber and thrust onto the bolt-face.

The WSSM hard extration issue is unfortunately a feature of the design and has been much discussed, the cartridge's 65,000 psi MAP peak pressures, short length and heavy brass unbdoubtedly factors as other posts say. I did see an interesting additional point raised by a well known and respected gunsmith / rifle builder on the Benchrest Central forum some years ago. The hypothesis was that with the fat case and chamber, there is insufficient metal left around the chamber in factory rifles at any rate. The receiver tenon and barrel expand excessively at peak chamber pressure thereby allowing considerable case expansion and after pressure reduces, the chamber contracts back to its original dimensions, but the case-body less so leaving it a very tight fit in the chamber and giving hard extraction.

It sounds reasonable, but how about the WSM / SAUM then with similar characteristics and pressure? Do their rifles have more metal around the chamber? If this is a key factor, heavy barrel custom rifles would have fewer extraction issues - does this actually apply?