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Preassure

Sako Collectors Club Discussion Forum

Webphut

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Hello,
So a ways back in a thread I put my foot in my mouth explaining to another person my interpretation of case pressure was. In a polite way , several of us in the thread were called out or stood corrected. Well I wanted to further learn my short comings on the topic of case head pressure and decided to dive into it and comprehend it. Fast forward now a few months of learning, I have come to think I understand how to measure for signs of high pressure. Please, I want to be corrected here by those who know more on the topic, so please fire away at me . I learned after a lot of reading a lot of know it alls threads on the internet and then one day an older gentleman showed me two things my father had always showed me, but never explained why. This older gentleman explained to me how to use my beat up old micrometer I had already been using to measure my case heads with and what to look for. He explained to me that I need to use a chronograph too. That’s all I needed to maximize the potential accuracy of my cartridges. I learned the importance of getting my fps for my shots as high as possible with the least amount of difference in fps between shots. AND to not exceed .0005” of case head expansion. Well what a learning experience. Now please chime in. I have found that this very small amount of information was life changing for my reloading. I think that there is common sense in everything. I also feel that there is a lot of older common sense that got left out of the conversations from generation to generation. Thanks to this gentleman, I have a whole new outlook on handloading. It has gone from what used to be frustrating to now an enjoyable challenge.
 
not exceed .0005” of case head expansion
Measuring case head expansion has long been a "fallback" effort to attempt to anticipate when sustainable pressures have been exceeded. Unfortunately, there are myriad problems with this method. But the old timers like the great Ken Waters didn't have access to copper crushers, much less piezo-electric transducers, so they used what little they had to guestimate pressure, fickle as it was.

First, case heads are rarely perfectly round. Think about it. For a half-inch diameter piece of brass to be out-of-round by only 5/10,000ths is nothing. Cases can have very small burrs or other imperfections which make it possible to measure a great deal more difference than that just by rotating the case a few degrees. The very highest quality micrometer used by an experienced, delicate hand would have a hard time discerning .0005" change -- even if placed precisely at the same place twice in a row on the case head -- which is unlikely to happen.

Second, even if .0005" were the proper "do not exceed" measurement for one size case head, it wouldn't be for another. There's a lot of difference in how much a given pressure can expand a .223 case head of .375" and a belted magnum of .532", not to mention that each has a different size hole in the head due to using different size primers.

Third, the difference in expansion due to case head size is negligible compared to case head hardness. Brass varies from brand to brand and even from lot to lot within a brand as to its alloy and elasticity. Obviously, harder brass will expand less than softer brass, and in addition is usually more elastic, so it springs back closer to its original dimension even if stretched more. Five ten-thousands of an inch in expansion (even if you could reliably measure it) would falsely indicate much more pressure with a soft case head than a hard one.

Cases also vary in construction. Some brands have a much thicker web than others. Weigh a few cases of different brands and you'll see from just the weight that some have extra brass -- and that brass is usually in the web. A thicker web, just like harder brass, resists expansion more.

All of these variables tend to render case head measuring a not-very-useful effort. However, I will stipulate that if you are experiencing measurable case head expansion with a given load with a given lot of brass then that load is "excessive" -- meaning unsustainable in that it will soon render the brass unusable -- and should be reduced. The identical load in different brass might exhibit more head expansion or no head expansion, depending on the brass.

Remember, the brass of the cartridge is much weaker than the steel of the rifle's action. Sure, rifle actions can be wrecked by excessive pressure, but it is always the case that fails long before the action will. What you strive for in managing pressures is keeping pressures within levels which are sustainable, meaning that your brass won't develop loose primer pockets by only the second or third firing, and further, that you don't experience a failure like a blown primer. It may not wreck your gun, but getting a face full of powder gas and maybe brass fragments is at the least unpleasant, and worse, may be personally injurious.
 
I second what stonecreek said & will throw my two cents in. Why do reloaders think they have to get "maximum" velocity & always try to push the limits of SAFE reloading. I've been reloading for over 50 years & have never, ever achieved good accuracy when using "max" loads. Without sophisticated pressure testing equipment there is no way you can determine what your pressures are by measuring brass, looking at primers, or "reading" any other, so called, pressure signs, no matter what BS you have been reading or have been told. Best accuracy is usually found 5 or 10 % below the listed max in any reloading manual & the loss of velocity is absolutely of no consequence with regard to trajectory or terminal energy levels with any cartridge. It just doesn't make any logical sense to push any cartridge to it's max. I think too many people read to many gun rags & think the extra few pounds of impact energy & the few 10ths of an inch flatter trajectory are going to actually make a difference in the REAL WORLD. Put your calipers away, step away from the reloading bench, & THINK about it. Does 50 or 100 fps really matter?
 
Thank you for the input. It is really fruitfull. I agree with cases being non symetrical and I do agree with you on how it probably isnt a perfect way to look for signs of high pressures, but given what I have and my lack of interest in pushing loads too far, it has been working preety good for me so long I document the measurements. I should also note that for the most part, my most accurate loads often are the ones that are not the max loads. Without pointing a loaded topic at myself here, I probably should have put into the original post that when I am aiming to have a minimum residual energy at the target location, a deer at 250 yards for example, I try to keep my velocities high enough to keep the desired energy level at that distance. I do not support long range hunting so please do not even get me started on any shots and residual energy past 300 yards. Most my shots are taken based more or less on how lazy I feel at the moment I could shoot. In other words, I often will just let a animal get as close to me as possible or as close to an area that is accessible by pack horses or maybe a Honda four wheeler and stalk up to it at that point then shoot.
 
I second what stonecreek said & will throw my two cents in. Why do reloaders think they have to get "maximum" velocity & always try to push the limits of SAFE reloading. I've been reloading for over 50 years & have never, ever achieved good accuracy when using "max" loads. Without sophisticated pressure testing equipment there is no way you can determine what your pressures are by measuring brass, looking at primers, or "reading" any other, so called, pressure signs, no matter what BS you have been reading or have been told. Best accuracy is usually found 5 or 10 % below the listed max in any reloading manual & the loss of velocity is absolutely of no consequence with regard to trajectory or terminal energy levels with any cartridge. It just doesn't make any logical sense to push any cartridge to it's max. I think too many people read to many gun rags & think the extra few pounds of impact energy & the few 10ths of an inch flatter trajectory are going to actually make a difference in the REAL WORLD. Put your calipers away, step away from the reloading bench, & THINK about it. Does 50 or 100 fps really matter?
Pardon my late response.
Mr, Paulson is correct, published data in reloading manuals are there for two prime reasons, #1 To guide reloading people to safe procedures and provide safe data & good results in reloading cartridges.
#2 To lessen / avoid the chances of being sued.
Following the directions set by the mfg. will in my opinion will assure a good safe outcome for all involved. Reloading can be a very dangerous task if not taken seriously. Just my 2 cents, be safe & have fun B/T
 
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