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Old January 22, 2020, 09:19 PM   #258
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Join Date: March 2, 2014
Posts: 7,447
Nicely done--your illustration does get to the heart of the unsupported case head issue. However--what's important to note is that it is because the SAAMI spec inherently allows a max 1.72 headspace as compliant the delta between that and the 1.714 of the ptg no-go gage is what allows the head area of brass like starline's to be potentially unsupported if the case backs all the bolt breech face upon firing. Using the ptg gage a user might be unaware of that since it could close on an autoloader--which is the case with my Faxon barrel and carrier. I suspect ptg's 1.714 no-go is what the SAAMI specs will eventually settle on.

Doing a cast should get you very close to the actual chamber dimensions, I think it would then be a matter of getting your total headspace and subtract the chamber case support area from the headspace and then take that result and then subtract the distance of the case rim face to the top of the case web. If the result is negative--then I think that would mean it's supported by the chamber. If that result is positive--that would mean part of case above the head could be unsupported potentially if the case rim face was backed all the way to the bolt breech face upon firing. That's what I think is going on with the loads I did using starline.

Why do I think hornady's brass is better being closer to the full 1.71? Again, just pure conjecture on my part, but the timing of the peak pressure and gas cycle in an autoloader with the 350 legend is a tricky thing--in part because of it being such a long strait wall cartridge. I know at least in the case of my Faxon barrel that's why they chose the short carbine gas length to reliably cycle in an 18" barrel. What I found was that depending upon when that peak pressure was achieved--and how fast the case sealed to the chamber--that had a pretty significant effect on gas leakage which in turn effects consistency. Of all the boxes of factory ammo I tested hornady's whitetail showed the least signs of gas leakage and usually the best consistence and SD numbers.They are powder wizards--so maybe that was one reason I got better results--but they also have significantly longer brass than the other guys. I think it does make a difference--but can't exactly empirically prove it.
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Last edited by stagpanther; January 22, 2020 at 10:09 PM.
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