counterclockwise
Member
- Joined
- Jan 17, 2008
- Messages
- 176
Which brings us to measuring case heads. This method was actually used by the reloading manual publishers for many years. It depends upon an experienced user with a blade micrometer, taking precise and repeatable measurements at the correct area of the case, and using the same lot of cases.
This is probably the most accurate method of checking for overpressure for most reloaders. It is still referenced in the ABC's for Reloading in the later editions. The hardness of the case head is probably not going to change over refurbishing/reloading cycles because it is already very hard and (under normal circumstances) unyielding. Bottlenecked case chambers typically pose the web of the casing (rear end of the case containing the charge pressure during firing) at the opening of the chamber. Chamber pressures that expand the case web radially beyond yield stress level should be taken seriously.
For the .308 case family of configurations ( .243, .260Rem, 7mm-08, .308Win, .358Win and .375Jag) there is an FEA analysis using LS-DYNA code at this website. It is worth going there and looking at the results. At the bottom of the report, the translation of residual expansion (post yield strain) after firing back to maximum chamber pressure is presented.
http://www.varmintal.com/a243z.htm
IMO, the problem with the primer flattening flag (primer cup assumes the shape of a .22 rimfire case) is that commercial rounds can end up looking like that. I have some fired rounds of .308 winchester hunting ammo that have severely flattened primers. Same goes for case head appearance. Take a look at some fired RSAUM cases sometime. I have photos where the RSAUM Remington ammo case head begins to extrude up the ejector pin hole. This causes some confusion in the judgement of the visual inspector.
That is a good set of photos presented up top. But, the lower right photo is not a overpressure sign. That is the end game we should all strive to avoid. I think the term "case head separation" does not do it justice. That should be "case head failure" or "case head structural failure". Damage of the firearm and threat to the user is assured in that situation. Superheated plasma at 50,000+ psi pours out the back, often times vaporizing the primer cup and anvil, dumping onto the bolt face, melting the firing pin tip, curling up the extractor claw, going on back and down inside the bolt splitting it like a log splitter, pressurizing the annular space around the bolt, and splitting the bolt carrier (in the case of an AR, pressurizing the aluminum structure of the upper receiver splitting it open, blowing down through the magazine bulging and blowing out the floor plate).
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