Rant: Another gun rag gets .308 vs 7.62 NATO wrong

[DWalt]

Back a few years I did a lot of shotgun shooting which equated to a LOT of reloading. I shot a K-80 in skeet with Briley and/or Kolar tubes. We shooters noticed quite a few split chambers in the 20 ga. Briley did some checking and found that a perfectly safe 20 ga load through the years had really high spikes in pressure when checked by PE testing. Lead crushers never read the very high momentary pressure spikes. They let it be known that we should switch powders in the 20 and 28 if loading Green Dot. . I used Unique or Herco in both and never split a tube. Just an illustration about the differences between crusher pressures and Piezo Electric testing

Technically, lead and copper crushers have a large time constant while piezoelectric gages have essentially none. The time constant is a measure of how quickly a measurement system reacts to an applied force. The crushers take a comparatively long time to react to a sudden step change pressure impulse and miss the peak. Piezoelectric gages react to rapidly increasing and decreasing pressure force nearly instantaneously in real time, and are vastly more precise.

 

[Dave_n]

LVSteve - thanks for good informative article. A shame so much BS followed it. Interesting question on the .223 VS 5.65 - if you have this comparison, would appreciate seeing what you turn up. Get the feeling the results are similar to the 308 VS 7.65 pressure measurements. Would be good to see all such data on a common base, but we can't seem to go from the old English measurements to Metric which is a better system for calculation. Again thanks for useful data.

When I first came to the US in 1968, there was a move afoot by GM to change nuts and bolts in cars to Metric sizes. Now go forward a short while and you had oil all over the floor as GM made their first change. Need I say they altered the drain plug in the sump to 25 mm. Too large for a 1'' wrench (Spanner) and the 9/8th wrench was too large for the bolt head. Since as a scientist I was very familiar with the metric system I simply laughed, but my US (non scientific) friends were "most upset". Even more entertaining was the lack of any reason even today not to use Degrees C rather than be the sole user of Degrees F left on the Planet!!! Dave_n

 

[ISCS Yoda]

OK lets all calm down and take it easy. The next thing to come up will be the age old.

View attachment 157241

You do realize that you can clip articles from a magazine, yes?

 

[WR Moore]

Once upon a time, the Speer website had an outstanding document on their website which explained both CUP v PSI and standard pressure v .38 Spl +P. Unfortunately, it not only disappeared, I didn't print out a copy. I've been kicking myself for decades over that.

From sometimes less than total recall: At low pressures, CUP & PSI are identical. Up to around 17K or so. At rifle pressures, there is a formula that can convert one to the other. At least where the copper pellet compression has been correctly established for that batch of pellets. IIRC, there was mention that .38 wadcutter ammo was used to confirm calibration of the piezo electric units after getting a CUP reading on that ammo.

In between about 17K and about 35-40K (?), there's no correlation. You get your pressure measurement from one or t'other and go with it. Unless there's been some discoveries since then.

Reaching way back to the OP, editors are rarely chosen for their technical expertise on whatever subject the publication covers. Ya gotta maintain or even increase the readership (and ad revenue). Decades later I still recall an article in a well regarded publication where the author gushed over a new firearms producer inventing a more accurate measuring system that used "mils". A "mil" is machinist speak for one one thousandth of an inch.

 

[rockquarry]

Once upon a time, the Speer website had an outstanding document on their website which explained both CUP v PSI and standard pressure v .39 Spl +P. Unfortunately, it not only disappeared, I didn't print out a copy. I've been kicking myself for decades over that.

From sometimes less than total recall: At low pressures, CUP & PSI are identical. Up to around 17K or so. At rifle pressures, there is a formula that can convert one to the other. At least where the copper pellet compression has been correctly established for that batch of pellets. IIRC, there was mention that .38 wadcutter ammo was used to confirm calibration of the piezo electric units after getting a CUP reading on that ammo.

In between about 17K and about 35-40K (?), there's no correlation. You get your pressure measurement from one or t'other and go with it. Unless there's been some discoveries since then.

Reaching way back to the OP, editors are rarely chosen for their technical expertise on whatever subject the publication covers. Ya gotta maintain or even increase the readership (and ad revenue). Decades later I still recall an article in a well regarded publication where the author gushed over a new firearms producer inventing a more accurate measuring system that used "mils". A "mil" is machinist speak for one one thousandth of an inch.

Editors are hired for their expertise at editing; gun knowledge is secondary but most have a pretty good grasp. Ideally, they buy articles from writers who know what they are talking about. It all comes together most of the time, but not always.

A good rule of thumb for writing a good article is to research your work enough to know more about your subject than 99% of those who might read the article. If you know less, you get into trouble with credibility. Look at YouTube gun stuff if you want to see numerous examples of this.

 

[DWalt]

I would suspect that ammo marked 223/556 is considered by it's maker to be safe to use regardless of the chambering specs and throat dimensions. I have used both interchangeably for years with no difficulty in quite a few different rifles.
Military chambers are often a little generous (to allow for dirty ammo) and may well have a longer throat but I really doubt there is a significant difference between commercial and military loads with the same weight bullets. I can see where a tight, match spec chamber in 223 could have problems with mil spec ammo but it could very well have trouble with some 223 commercial stuff as well. As more than one poster already mentioned, pressure measuring methods vary and it's not easy to relate one system to another.

As I earlier mentioned, an M16 will hold together at a chamber pressure up to at least 100 Kpsi. Problem is that piezo pressure gauges do not work very well beyond around 85 Kpsi. They may be able to today, But when I was doing this stuff at the Hercules ballistics lab over 50 years ago, that was the upper limit. We were taking Polaroid pictures of vacuum tube oscilloscope screens to get P-T curves. That is how primitive things were then. I don't think microchips had even been thought of at the time.