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Old June 16, 2009, 06:47 AM   #1
micksis86
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High pressure signs in rifle handloads``

Hi,
This is my first post so thanks in advance for any help you guys can offer.
I've just began handloading and all is going very successfully thanks to plenty of research. However during my research i've come across very little difinitive information regarding signs of high pressure in handloads. There is alot of speculation but i want to know what indicators are the best and most reliable to go by when trying to detect high pressure.
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Old June 16, 2009, 07:37 AM   #2
SL1
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Micksis86,

There are a lot of rifle cartridges with maximum pressure limits that range from about 15,000 psi to 65,000 psi. Some rifles can stand higher pressure than some of the cartiridges they shoot, and some cannot.

Pressure signs that can be watched for include (in approximate order of observation with increasing pressure): case wall pressure ring diameter, primer cup flattening and imprinting, case head diameter expansion, case head imprinting with breach face features, case rupture.

Depending on the strength of the GUN, the gun may "fail" structurally before you even flatten primers if it is designed only for old black-powder cartridges. Or, a modern bolt-action rifle MIGHT hold together over 90,000 psi, when the cases are obviously damaged in the head region. So, one pressure sign does not serve to warn of unsafe conditions in all situations.

Learning exactly WHAT to watch for and HOW to look for it is actually pretty involved for any of these methods, because none of them are very accurate measures of pressure, and you need to calibrate the observations to the pressure in YOUR gun.

A full treatise on all of these methods would fill a substantial book, so we can't cover everything here. And, there has been some MIS-information printed about them, too. So, you really need to study more than one source to avoid being mislead.

If you tell us what gun and cartridge you are shooting, we can get a more specific discussion going that would apply to your situation.

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Old June 16, 2009, 05:42 PM   #3
micksis86
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Well i've began reloading for my remington 700 SPS in .308 so information spcific to that would be great.
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Old June 16, 2009, 06:44 PM   #4
rgitzlaff
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And now to give you some kind of an answer to your question... some general pressure signs to look for on your brass:

1. Degree of primer flattening. This isn't necessarily the firing pin dent being flattened. Pay more attention to how flat the radius gets. When you load a new primer, look at the gap left by the outer radius. Compare this with a fired primer. See how the fired primer is flatter and has a smaller crevice from the radius? The higher the pressure, the flatter the primer will be.

2. Look for indentations with a crescent or circular shape on the base of the case. This will be made from the brass flowing into the ejector or extractor pocket from the high pressure. If you are seeing this, your pressure is for sure too high.

3. Case cracking around the base can also be indicative, if the brass is not too old.
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Old June 16, 2009, 09:27 PM   #5
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Measureable pressure signs

If you want some pressure signs that you can measure, learn about the pressure ring and the case head diameter measurements.

The pressure ring is the part of the case wall a little forward of the case head that expands upon firing and stays expanded more than the rest of the case. Its diameter increases with the peak pressure of the load, at least up to a point, where it may stop increasing. For a .308, the SAAMI max pressure may or may not be above the point that the pressure ring stops increasing in your gun. It depends to some degree on the chamber dimensions. Of course, if you don't resize the case after each firing, then the pressure ring will only be useful for the first firing. If you do fully resize it, then it will come back to about the same diameter for the same pressure load the next time. But, as more loadings are fired and the case repeatedly resized, the brass work hardens and expands a little less, making it LOOKs like the pressure is decreasing. So, you need to work with brass that has only been fired a few times to use this method. To take the measurement, use a micrometer and measure the case diameter at several points around the circumference. I try to take the maximum and minimumn readings on a case and then use the mid-point between the two values, because that is the measurement that I can reliably reproduce.

The case head expansion method looks at the diameter of the solid part of the case, just ahead of the extractor groove. This technique is somewhat different from the pressure ring method in that the case head expands ONCE by a particular amount at a particular pressure, and will not expand again until that pressure is SUBSTANTIALLY exceeded in a subsequent load. To measure the case head diameter on a .308 case, you will either need a blade micrometer to avoid interference of the micrometer anvils on the rim, or you can use a regular micrometer and file two opposite sides of the rim just enough to clear the round anvils on the standard type micrometer.

I like to use these two methods because they can give me some warning that the loads are getting pretty hot BEFORE I reach the over-pressure conditions that result in sticky extraction and primer pocket loosening.

To learn how to use these two methods, you need to start looking at these two measurements with factory loads in YOUR gun. See what a variety of factory rounds measure in your gun. Then, as you work up handloads from starting charges toward max charges, you should start seeing patterns for the changes in these two measurements as you approach factory pressures. Your handloads may go above factory pressures as you reach max charges in your manuals, and you will see how these case measurements change as you reach max.

Just remember that it is hard to get an accurate pressure reading with these techniques. You are measuring how your brass is being affected, but that might be different for different brands of brass at the same pressure. But, in your gun, the brass is probably the limiting part. So, you can probably set a limit on the case head expansion that you will accept for the first firing of a new case that is reasonably close to the SAAMI limit and not be too worried if some lots of brass get you a little over. (It would be a different situation if you had a weaker gun and were trying to avoid damaging the gun by looking at the pressure ring diameter for say a .45 Long Colt, which may not be able to take 25,000 psi.)

If you are not trying to push the limits, then these two methods can give you some good indications of how your pressure in changing with load development. But, if you are going to try to push those limits, you would really need to learn a lot more about these methods to make sure that you aren't being mislead by some of the intricasies so that you don't push into unsafe pressures.

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Old June 16, 2009, 10:25 PM   #6
Crosshair
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If you need a 5lb mallet to get the bolt open, that might be a sign of high pressure. Extraction problems are a very strong sign of high pressures.

Some lever guns, like the Marlin 336, develop extraction problems well before the gun lets go.
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Old June 16, 2009, 11:09 PM   #7
RGS
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Pressure signs

By the time you see excessive pressure signs, you've already gone too far. In good modern bolt action guns, this usually isn't the end of the world. Just cut back on the powder until pressure signs go away.

You can be significantly over max pressures Without any obvious excessive pressure signs

Are you using a good handloading manual or manuals? Use that information and see how long you brass lasts. If you have to trim cases every 2 or 3 loadings, chances are you need to cut back on the powder a little. If you can get 10 loadings in a case before it thins out at the case head, you are probably not terribly over loaded. Keep good records of each batch of brass you use so unusually short case life will get your attention.

If your .308 suddenly recoils and sounds like a .300 win mag, you may want to investigate your load data no matter what the fired case looks like.
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Old June 17, 2009, 09:47 PM   #8
micksis86
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Thanks,
That's some really good info.
I'm using Lyman's #49 and Speers number 13 reloading manuals so i think they're reasonably reliable.
Thanks again for your help,
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