Forming brass for 8mm


January 25, 2011, 08:15 PM
need to make up some fire forming loads for an 8mm gibbs. This will mean expanding the neck of 30-06 brass to bigger than .323 then sizing with 8mm gibbs dies. I know that I need to form a false shoulder on the cartridges to provide a good crush fit when forming in the chamber. My question is: Is running the case through a 338-06 sizing die going to give me enough of a shoulder or should I look into a set of 35 whelen dies? any other suggestions would be helpful. Thanks.

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January 25, 2011, 09:59 PM
I'm not sure what your chamber neck actually measures, but I'm guessing it runs about .006"larger than the case neck which should run around .354" for an 8mm using a .323 bullet.
338-06 case neck should run .369" after sizing, and should be enough to give you the false shoulder based on a .360" chamber neck diameter.

These are all assumptions based on case drawings and there's a few "should" tossed in the mix. It might be beneficial to buy both sets and resell the one not necessary to be on the safe side.


January 26, 2011, 03:16 PM
I would do it two ways, my favorite case is the 280 Remington because the shoulder is ahead of the 30/06 by .051 thousands. After forming the case must be necked down to 8mm before they will chamber.

I would neck up the cases to 338 or 35 does not matter, after necking up I would adjust the die off the shell holder to avoid forming the new shoulder back more than necessary, when using the 338 or 35 Whelen die to form the second shoulder I would adjust the gap between the top of the shell holder and bottom of the die to .110, this adjustment should allow the die to form the case with no head space in the perfect world. I do not live in the perfect world so I would start with a gap of .120 thousands and not expect the cases to chamber, from .120 thousands gap I would reduce the gap in .005 thousands increments until the case chambered, if after forming the case it does not chamber because of resistance to bolt closing I will increase the gap and continue until the case chambers, THEN! I would verify the gap and record the measurement for future reference.

Using 280 cases or 30/06, when necking up the case will shorten the case, when firing the case the case will fill the chamber, when this happens the neck is pulled back and shortens again, from start to finish the case will shorten .035 thousands if 30/06 cases are used If 280 Remington cases are used the additional length of the 280 will off set the shortening of the case and I am a fan of covering the chamber with the case, all of the chamber or as much oof it as I can, I do not like my chamber to be exposed to the hot high pressure metal cutting of the escaping gas.

And no this method has nothing to do with bumping the shoulder forward or back it is about erasing the shoulder, the case body becomes part of the shoulder and part of the neck becomes part of the neck.

The last process in forming is sizing the neck to 8mm using the measurment when the gap was verified.

And I do not use reduced loads, when fire forming I do it once, I have formed cases with loads that some thought were horror stories or a more milder term called scary stuff but for those time as a factor is something they know nothing about.

F. Guffey

Vern Humphrey
January 26, 2011, 03:46 PM
I shoot the .35 Brown-Whelen in a custom '03 Springfield. My way of making brass is to load a case with about 10 grains of Bullseye, with a quarter square of toilep paper pushed down to keep the powder close to the primer. No bullet.

The Springfield, with its Mauser-type extractor, holds the head against the bolt face. I pull the trigger, work the bolt -- and out pops a beautiful .35 Brown Whelen case.

January 26, 2011, 09:45 PM
Thanks for the 280 tip. Think I'll stick with '06 brass cause I got about 400 once fired cases, and I don't need that many to keep my 30-06 happy....

January 27, 2011, 09:17 AM Caliber Reloading Data Springfield

F. Guffey

January 27, 2011, 03:53 PM
Vern, the 35 degree shoulder will lengthen the case body and shorten the neck, even then the neck/shoulder juncture on the 35 Brown Whelen is only .021 thousand different when measuring from the head of the case to the shoulder juncture, that is about the same difference between the 30/06 and the 30/06 Ackley Improved, it is said there is not a head space issue when firing 30/06 cases in the Ackley improved because the neck is shorter on the 30/06, the neck is sized when it is chambered and forms the rest of the case when fired.

This means the extractor does not have a lot to do when firing Ackley cases, back to Hatcher and his modified chamber, he moved the shoulder forward .080 thousands and fired 30/06 cases, the case head spaced on the extractor groove, his method could be tough on extractors and again 8mm57 ammo has been fired in an 8mm/06 chamber, if anyone took the time to measure that is .127 head space, others have fired 308 W in a 30/06 chamber, that is .388 head space if no one considers the 308 wedges into the 30/06 chamber at the shoulder because it is .011 larger in diameter than the 30/06 when measured from the head of both case to the same juncture on the case.

When forming 8mm Gibbs the body/shoulder is formed .202 thousands ahead of the 30/06 case body/shoulder juncture, and that is the reason I explained how and why I would neck the case up and then size the neck to get it where I wanted it.

I did not say I would not do it another way but if I had to explain why I would not fill the case with cereal.....

It is not possible to have it both ways, the standard thinking among relaoders is,... the firing pin strikes the primer THEN the shoulder of the case hits the shoulder of the chamber and in their thinking they never add time as a factor but their logic says the firing pin drives the case forward.... then we have to add another event to the firing pin drives the case forward, the added event would be the case extractor groove head spaces on the extractor....

I do not use reduced loads as in fast powders because of the rapid expansion of the hot high pressure gas, the rapid expansion of the gas has to be violent and I am not convinced the rapid expansion of the case is good for it.....I form first then fire and I am a big fan of reducing all that case travel.

F. Guffey

May 11, 2013, 04:18 AM
save the headaches of split necks , false shoulders , necking up to neck down , just run 35 whelen brass (brownells had some a while ago) straight into your gibbs dies ,load your rounds with normal loads (8mm06 data works well) and fireform , you will find hardly any drop in accuracy , you can target shoot and hunt with your fireform loads .
if you want any other info let me know .

john wall
May 11, 2013, 11:16 PM
What Waverace said. I had a 25 Gibbs in another life, used 270 trimmed to length to get the false shoulder.

The bolt should be hard to close on your brass.

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