Editor’s Note: This article on .223 vs 5.56 comparisons is an excerpt from Gun Digest 2013, the world’s greatest gun book.
.223 vs 5.56: Measuring Pressure
Before we get too deep into this, you also have to be aware of a change that happened in our lifetimes (well, the lifetimes of the old farts among us), and that is the change in pressure measuring. If you have an older reloading manual, you’ll see the measuring units denoted in C.U.P., and in some older manuals “CUP” and “PSI” are used interchangeably.
The old way of measuring pressure was known as the copper crusher method. In it, a test barrel would have a hole drilled through it to a specified set of dimensions. Then, a little copper cylinder was clamped in place over the hole. When the round was fired, the copper cylinder got hit with the pressure and was compressed. By measuring the length of the cylinder before and after, ballisticians could determine the peak pressure. This was known as “copper units of pressure,” or CUP, but was often expressed in pounds per square inch, or PSI. The copper (and lead cylinders, used for lower-pressure calibers) could only tell us what the peak pressure was, however, not how fast its onset was, how long it lasted, etc.
Today, transducers, or strain gauges, are used to measure pressure. Here, the gauge, which is essentially a transistor (it is more complicated than that, but we’re discussing firearms, not electrical engineering) is fastened to the barrel. When the gauge is stressed, the electrical resistance of the gauge changes. The beauty—and the problem—with this method is that it is dependant on a computer or other recording device. Depending on how much you spend, you can record the pressure of the event hundreds, thousands, or more times per second. This caused problems in published loading data.
Let’s construct our own cartridge, just so we can remain theoretical for the moment. The “.30 Zoomer Magnum” has a maximum average pressure (MAP, or the allowed peak) of 50,000 CUP. We use the newfangled transducer to measure the standard reference load (in this case, 42 grains of “XYZ” powder under a 183-grain soft-point) and come up with 57,000 PSI. The “new” MAP for the .30ZM is now 57,000 PSI, where before it had been 50,000 CUP. But the actual pressure has not changed, we are simply using a new yardstick to measure it with.
Then we run into problems. In checking loading data, we find that some of the data wasn’t as “clean” as we thought. An example: using “123” powder under the same 183-grain soft-point, we had found that we could get 100 fps more and still only see 50,000 CUP pressure. With the new transducer and seeing things in thousandth of a second slices, we see that, yes, the main pressure peak is only 57,000 PSI, the allowed max by the new yardstick, but we also see a second, higher, spike from the bullet hitting and stalling in the rifling. That spike comes in at 63,500 PSI, well over the maximum allowed. So, we have to throttle back the load data, and all of a sudden “123” powder loses its 100 fps advantage.
The problem came from the copper cylinder not being sensitive enough to register the second, over-max pressure spike, so, no, we have not “slowed down the load data to satisfy the lawyers.” We didn’t know we were going over-max before. We do now, and we have to adjust the data. (Oh, and just to add to the confusion, where you place the transducer can also have an effect on the pressure you measure.)
The SAAMI-spec pressure ceiling, the MAP allowed for the .223, is 55,000 PSI. No, there is no handy-dandy formula that lets you convert old copper-crusher pressures to PSI. The ballisticians tried, and they tried really hard, to come up with a conversion factor. The trouble they ran into was that every cartridge seemed to have its own factor. It was bad enough converting from CUP to PSI, but trying to tell people (and this is just an estimate, don’t use these as numbers to go by) that where they could use a plus-12 percent CUP-to-PSI factor for the .293, the .34-06 used a plus-15 percent, and the .305 used a plus-nine percent. (And, yes, I deliberately used nonsense calibers. Don’t try to decipher them, there is no pattern, nor any useful info beyond what I just told you.)
There was no way to formulate an equation for a “universal translator” of CUP to PSI. Give it up, forget the conspiracy theories your gun club buddy tells you, just accept the new info for what it is.
The NATO spec for 5.56 has a higher “ceiling,” but it’s also measured slightly differently, and, again, there is no handy-dandy conversion. The SAAMI method measures pressure at the middle of the case. NATO (the European measuring group is known as C.I.P.) measures at the case mouth. A CIP-spec 5.56X45, measured at the case mouth, shows a pressure of 62,000. Measured at the case middle, as SAAMI does, it shows 60,000 units of pressure.