Measuring Pressure

1

10vpossible

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I'm just about to start loading for a .22BR using Lapua brass, I've got a digital
mike which reads to 0.00005" what would the forum consider to be the maximum expansion of the new brass to be safe?
 
It's quite possible that you're a lot more patient and better at using a mic than I am, but I've found that measuring case heads as a method of determining pressures is a pain in the tail. If the chamber is cut to minimum specs for Lapua brass the cases also might not expand much.

In my experience stiff bolt lift is an easier method of determining excessive pressure and is due to case expansion. Flat primers depend on too many other factors to be used solely, but primer pockets that open up prematurely are a good indication of pressures that are too high. With Lapua cases that would mean that you've gone WAY over the top since Lapua case heads tend to be harder than the hubs of Hades.

That all being said if case heads (rims, extractor grooves, or solid head of the case) expand 0.0005" on the first firing, but don't continue to expand with that load the load is likely okay.
 
I use casehead expansion as my guide. In fact it's the ONLY guide I truly trust.

But it's easier than this.

Pay special attention whilst you seat your primers and you don't need to mic them. When the primer pockets loosen you're TOO HOT!!! Back down. Back down until the pockets remain tight for repeated loadings. I've got BR cases fired 100times that still hold primers tightly.

I use blades and flats both when measuring caseheads. I'll mic a casehead above, below and in the extractor groove and write it all down. Try it, work up your loads monitoring as you go and I'll wager that you'll find that on your 22BR you'll begin to experience loosening at about .002 growth using Lapua brass.

Expansion up to .004 will show an increase in looseness and beyond that you risk leaking primers.

I've recently been playing with this case in the 6X47L iteration and have found some cases which will still hold a primer all the way up to .006 expansion but this is a first for me.

BUT YOU DON'T HAVE TO DO ALL THIS.....

Just monitor your seating force. If your primers are loosening, BACK DOWN and you'll never have a problem.

For those case which HAVE loose primers...... it's your call. I touch them with fingernail polish or Bob-N-Roys Primer Sealant and load them up for varmints. Shoot 'em one last time and lose 'em. Doing this you DO risk gas-cutting your boltface. I haven't leaked one for 10-12yrs but I've gotten perty good at gauging "too loose for comfort."

Throw away the too loose cases. You can't fix them. Plus, they'll no longer go into your shellholders and will jam in the chamber if you're tight-chambered.

I SHOOT HOT..... like PPC style hot...... AND I want my cases to last forever so here's what I do. I fireform at least once using fast powder and light bullets. IMO this slams, settles and "sets" the casehead while making it harder. Over the next few firings the cases will harden up more. In a perfect world cases will expand just a thou or two while settling in, just enough to get rid of the raspy 'clik' as you seat the primer, yet remain tight, forever.

Now keep the loads below this expansion point and you're gooda'go.


hth


al
 
I have gone to the range with cases loaded with increasing amounts of powder and a blade mic capable of measuring to one 10 thou.

I measure the case in the extractor groove... rotating and recording the largest measurement. Then I fire that case and measure again. I do this one at a time with the increasing loads until I get expansion. The load that expands I consider too hot and I will use a lighter load.
 
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After using blade mics to check cases and chronographs to keep tabs on the velocity as indicators of pressure, I have also narrowed it down to loosening of the primer pockets.

The easiest way for me to determine it quickly at the range has been to take my arbor press and decapping rod/base and knock the primer out after firing the increased charge. It will take a small amount of steady force to remove the primer from a normal load, but if the primer practically falls out after the initial push, or you can unseat it with finger pressure, that brass is done. I usually fire the next charge in 3 shot groups when working up, and if groups hold, or get better and the pockets are tight, I keep going. If the groups open and pockets are tight, keep going, as your probably not at the upper level yet. If the pockets are getting loose and its shooting great, you will have to determine if that is where you want it because eventually you will be going through lots of brass.

The same load/pressure that is fine with one brand of brass just might ruin another brand, so its not really even a question of pressure in itself, but how much pressure can the brass handle before it becomes unuseable.

Lapua is the best. Accept no substitutes.:D
 
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I may be wrong about this, but I read somewhere that when mike-ing case expansion, you should use as a baseline a case that has been fired with moderate pressure -- not virgin brass.

Then the cases you are concerned about -- possible high pressure -- would be compared to that baseline case, not a virgin case.

Am I right about this, or all wet?
 
First, I know everyone has their..............

preferences, but I would sell or otherwise get RID OF the digital micrometer. They do have a tendency to get erratic :eek:for a number of reasons; I was told once they were "paranoid", as they got to a point where they'd give differing readings and, "they(the mic)never knew why" :D. Personally, I find it easier to trust the mechanical mics, but, that's just me. Anyway, I have found in some cartridges, you may get to a point where you may have some pressure problems in a smaller capacity case that's full or almost full, but when you switch to a slower powder, just when the loads are beginning to show promise, you run out of capacity :mad:. Back off about 5-8% and switching to a magum primer is sometimes effective, as it will keep your pressures down in most instances, allowing you to work back up, using the better, or slower powder in that particular case. ;) HTH
 
Montana Pete said:
I may be wrong about this, but I read somewhere that when mike-ing case expansion, you should use as a baseline a case that has been fired with moderate pressure -- not virgin brass.

Then the cases you are concerned about -- possible high pressure -- would be compared to that baseline case, not a virgin case.

Am I right about this, or all wet?
Click to expand...

I agree totally with this. In fact, to me virgin brass is just a lump of clay waiting to be molded into something useful. As it comes it really can't be measured for anything meaningful.

al
 
There was a gadget --- invented by one of the wizards at Border Barrels that consisted of an accelerometer and an electronics package ... the accelerometer was made to bolt on to a rifle scope (convenient attachment point). When the rifle was fired, the electronics package found the maximum point in the acceleration signal, and by having been fed the weight of the rifle and some dimensions, the maximum chamber pressure was calculated. Something like that. The theory is sound, but I have never heard of first-hand experience save from the inventor. Anybody ever tried one?
 
The right way is to use a Pressure transducer and record the data on an O-scope, that is the way we measured pressure when I worked at McDonnell Douglas. With the data that is recorded you can find the rise time, the peak pressure and the time of the pressure until the bullet leaves the barrel. I know most people don’t have this type of instrumentation in their home so you use what you have and do an approximation. Again when you measure the outside diameter of the case you get a ballpark figure.

john
 
Fla mac said:
The right way is to use a Pressure transducer and record the data on an O-scope, that is the way we measured pressure when I worked at McDonnell Douglas. With the data that is recorded you can find the rise time, the peak pressure and the time of the pressure until the bullet leaves the barrel. I know most people don’t have this type of instrumentation in their home so you use what you have and do an approximation.
Click to expand...
http://www.oehler-research.com/model43.html
http://www.oehler-research.com/ism83.html (industrial grade system)
http://www.shootingsoftware.com/pressure.htm

Such systems are used by small ballistics labs, or ones unwilling to deal with transducer systems. They are robust, and fairly reliable, and almost affordable for a home hobbyist.

Re: Case head expansion as a indicator of pressure. One engineer (not me) thinks it's unreliable. See his analysis at http://www.shootingsoftware.com/ftp/dbramwell july 19 04.pdf ,then decide for yourself.
 
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