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From the dawn of the age of smokeless powder the industry has needed some method for judging the safety of ammunition. For much of the time that method has been with the use of a copper crusher.

A copper crusher is a small cylindrical chunk of copper with controlled dimensions and hardness. The maker of the crusher calibrates them by applying a controlled pressure and measuring the change in length that results. Hence the “crush” nomenclature. Using that information they prepare a tarage table so the user can measure the length of their fired crusher and obtain a pressure value. For years that was reported as pounds per square inch. It really wasn’t a precise value and the industry knew that but since it was all they had it didn’t matter.

When the piezoelectric transducer method was developed that was recognized as a true measure of pounds per square inch (psi) and the term CUP (copper units of pressure) was coined to differentiate between the two. A tarage table shows a pressure number for each thousandth of an inch. The problem is that each thousandth can be as much as 300 cup.

A piezoelectric crystal produces a measureable current when subjected to pressure and that current can be captured electronically. Transducers are also calibrated in a hydraulic systemand that information used to adjust the electronics to display a value.

The barrel used for crusher measurements has a small hole on top that is located approximately in the middle of the case. A gas check is inserted and then the crusher and the whole thing is held down by a clamp. Many loading manuals have pictures of both setups. Transducers are shaped to fit the case (conformal) and screw into a threaded hole in the barrel.

One of the flaws with crusher measurements is that things are happening so fast that there is a time lag in the response of the crusher. What you get is something less than true peak pressure and this accounts for the difference between crusher and transducer measurements of the same round.

I think it is now accepted that with many cartridges peak pressure is reached before the bullet even starts to move (bodies at rest want to stay there Mr. Newton said). With most handgun rounds it certainly is past before the bullet leaves the case and with slow burning rifle powders by the time the bullet has traveled just an inch or two.
 

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Charlie Petty said:
...The barrel used for crusher measurements has a small hole on top that is located approximately in the middle of the case...
Curiosity: Is the case pre-drilled to match this hole, or is a solid case allowed to rupture in order to affect the crusher cylinder?
(I have to assume that, were the case pre-drilled, the case would be inserted into the barrel, a feeler used to properly locate the pre-drilled hole, and only then would the crusher device be clamped into place. Am I correct?)
 

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Steve M1911A1 said:
[quote="Charlie Petty":2nsbj7w7]...The barrel used for crusher measurements has a small hole on top that is located approximately in the middle of the case...
Curiosity: Is the case pre-drilled to match this hole, or is a solid case allowed to rupture in order to affect the crusher cylinder?
[/quote:2nsbj7w7]

As I understand the process, at the moment of firing the case obturates the breech and becomes a near liquid under pressure, whatever part of the case under the pressure sensor is measured at that point. What you get with the crusher is the peak pressure at that point of measurement. If there was a hole in the case, strange things would happen. This is the source of case bulges and is why brass is annealed. Military ammo in this country is usually annealed after polishing so the quality folks know it has been done.

I have problems with my little Ruger .22 SP101, the high pressure ammo cases expand more than the standard velocity cases and some of the chambers are a bit loose at the base.

Geoff
Who supposes he should send it back to Ruger...but I've been told they won't replace it with a .22 DA revolver. :ek:
 

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Discussion Starter #4
In normal use cases are not drilled in either method, but there are situations where they are. When this is done the case is held in a jig so the hole is drilled in the right place.

There is some debate but even though drilling might give a slightly more accurate reading it isn't worth the trouble in a production setting. One exception is with 50BMG ammo which is routinely drilled because of the thickness of the case.

Cases fired in crusher guns end up with a hole where the crusher was anyhow, but there is only a slight circular mark where the transducer contacted the case in the newer system.
 

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My thanks to both of you.
Your explanations were very clear.
 

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Steve M1911A1 said:
[quote="Charlie Petty":2banjy4v]...The barrel used for crusher measurements has a small hole on top that is located approximately in the middle of the case...
Curiosity: Is the case pre-drilled to match this hole, or is a solid case allowed to rupture in order to affect the crusher cylinder?[/quote:2banjy4v]I believe the European (CIP) pressure measurement standards call for use of a pre-drilled case, and call out specs for both the position and size of the hole. (They use piezoelectric transducers to measure the pressure.)
 

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I was under the impression that CIP did pressure readings at the case mouth.
 

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Wikipedia says:

Under C.I.P. proof test standards a drilled case is used and the piezo measuring device (transducer) will be positioned at a distance of 25 mm from the breech face when the length of the cartridge case permits that, including limits. When the length of the cartridge case is too short, pressure measurement will take place at a cartridge specific defined shorter distance from the breech face depending on the dimensions of the case.

Under SAAMI proof test procedures, for bottlenecked cases the centre of the transducer is located 0.175 in behind the shoulder of the case for large diameter (0.250 in) transducers and 0.150 in for small diameter (0.194 in) transducers. For straight cases the centre of the transducer is located one-half of the transducer diameter plus 0.005 in behind the base of the seated bullet. Small transducers are used when the case diameter at the point of measurement is less than 0.35 in.

The difference in the location of the pressure measurement gives different results than the SAAMI standard.[5]
 
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