Posted: 2/10/2016 9:27:07 PM EDT
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Read an interesting article on AccurateShooter.com about using a blade micrometer to measure safe case head expansion. Referenced an article on IMRpowder.com under tips and tricks. All this to the effect that most pressure signs were fairly unreliable in determining hot or max loads, ie primer changes, swipes, etc, but measuring the case head was a reliable sign of a safe load vs. overpressure. Depending on the caliber, there seems to be a "standard" safe case head expansion.
I did a google search and there was a thread on LongRangeHunting about this and one fellow referenced an engineering article that compared case head measures (2 types) with a strain guage method and concluded that measuring case heads is not a good method for determining hot, overpressure, or max loads. So...finally, my question for you guys that have done this for a while is what have you found to be a reliable method for determining safe loads? Anybody measure case heads on a regular basis...other than in with a case guage? |
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Quoted:
my question for you guys that have done this for a while is what have you found to be a reliable method for determining safe loads? When the primer pockets blow out in under 10 reloading, the pressures are TOO high. When the primer pockets survive 30 reload cycles, the pressures cannot be TOO high. |
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Quoted: When the primer pockets blow out in under 10 reloading, the pressures are TOO high. When the primer pockets survive 30 reload cycles, the pressures cannot be TOO high. Quoted: Quoted: my question for you guys that have done this for a while is what have you found to be a reliable method for determining safe loads? When the primer pockets blow out in under 10 reloading, the pressures are TOO high. When the primer pockets survive 30 reload cycles, the pressures cannot be TOO high. This, but for a quicker assessment - assuming headspace is snug at -0.003" or less and the ejector/extractor recesses aren't sharp - one or any combo of excessive primer cratering, flattening, or excessive extractor/ejector marks is my cue to stop and re-evaluate the load and maybe the firearm. |
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Determining safe pressures based on the deformation of metals is problematic because the metals you're testing on aren't in a consistent state before each test so there's nothing to really measure. Case head expansion just like primer reading and looking for ejector swiping and all the rest of the signs of excessive pressure are all just signs. They're not measures, just indications. They're more accurate than chicken bones scattered across a plate but less accurate than a copper crusher or piezoelectric transducer.
You use them all. If you get a sticky bolt lift you're probably getting excessive case head expansion. |
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Quoted:
RegionRat, Please tell us more about a strain gauge. I shoot an AR. I don't shoot loads that I know to be hot, I am more interested in accuracy. It's all about finding the nodes. Here is a consumer turnkey system that comes with everything you would need short of a laptop. A strain gage is placed on the chamber outer diameter and the signal conditioner feeds a laptop style output. Getting one mounted on an AR isn't easy due to the bbl extension, but it isn't impossible. A method of getting around mounting on an AR is to use a sister bolt action for load testing, where the sister is one made of an identical bbl blank and chamber reaming but in a bolt action where testing is easier and safer. If you are experienced with load development and not getting into radical bbl lengths, i.e., very long or very short, it shouldn't be too difficult to work with known accuracy loads for ARs. There are dozens of good recipes for a Service Rifle that can be safely tuned to a slightly different rig without going into the Hot Zone. Getting accuracy out of your AR would be based on the same methods used in the other accuracy disciplines, Ladder, OCW, etc, but made only slightly more demanding on you because of the use of a semi-auto. Are you playing with specialized bbl styles? Or are they close to a standard 20" target bbl like a WOA or CLE? https://www.shootingsoftware.com/pressure.htm |
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The problem with using a another gun to check pressures is chambers are different enough to make the info useless.
With my old bbl on my 700 I could get to around 68k psi before I would get hard bolt lift and swipes. The new barrel I cannot get anywhere near that pressure due to hard extraction (but easy bolt lift). I had to drop all my charge weights down a couple full grains( same brass ). If you look carefully at the primers radius there is a difference that you can see usually if you are getting too hot as compared to unfired. When I do workups I observe the radius as I go on up to max pressure, I then store this info in my head for later use(some call it gaining experience). smiley emoticon I have pictures of cases I fired at various pressures to show primer flattening. I can post them when I get to a PC. |
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Here is a good example why you cannot always trust a load that works fine in a different rifle.
These are the exact same load shot at the same day, same time. The left column was fired out of my 18" WilsonCombat match bbl the middle column is from my 7.5" DPMS KittyKat bbl the far column is of course unfired. notice the radius on the primer(Wolf) the middle is flattened pretty good but I would consider ok but the left is waaay too hot for that rifle. 
Here are a couple with an obvious difference, these are from my bolt gun on which I have a strain gage to get the pressures. Fed210M primers(rem700, all loads will crater no matter how light BTW) 
Here is one more subtle with various pressures listed. Harder to really tell but the 66k has the smallest corner radius on the primer. 
My pressures are based off my PressureTrace readings calibrated based off MVs, load data and tuning QuickLoad, so these actual numbers may not be exact but it is very reliable for comparing one load to another fired from my gun the MVs are from a 20" barrel(.308). Just below is a trace for 168gr Hornady Match ammo. 
This is my 175gr handload.Testing Wolf Primer vs. 210m 
Here is one with a long delayed ignition on shot 10, the primers goes that is the 8k line then .3milliseconds later the powder ignites. A too hot load too. 
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