There has been a considerable degree of discussion lately about PRE (pressure ring expansion) measurements lately and what they mean.

I performed some analysis to attempt to shed some light into this darkly gray area. Consider for a minute that the case cannot expand more than the rifle's barrel; however, due to the great likelihood of spring back the measured PRE is most likely less than the actual expansion of the rifle's barrel during the firing event. Read that ten times or until it makes some sense.

The proceeding analysis assumes NO spring back of the case and that the measured PRE IS IN FACT equal to the actual rifle barrel deflection during firing. These assumptions, almost without doubt, will provide allowable pressures which are higher than can be achieved!

The maximum allowable pressures (in the red highlighted rows) correspond to the point at which the rifle barrel expansion equals the lowest likely barrel yield strength (around 108,000 psi). Again remember that due to the assumptions this pressure is HIGH and should be reduced by ~ 10% or more for safety. What does this mean you ask? It means that at the pressure if the red highlighted row is achieved the barrel will permanently BULGE! Repeated firings at or near these levels will rapidly result in a blown barrel!!

Having said all that here is the data.



The red highlighted pressures are:
M70 - 80,800 psi
Ruger M77 MkII RSM (Magnum Safari action) - 73,500

Chart number two:


The red highlighted pressures are:\

CZ550 (remember 500 Jeffery case) 54,900 psi
Winchester M1894 Big Bore (450 Marlin case) - 56,900 psi

They are based off of standard chamber diameters (which will vary slightly from rifle to rifle) and production barrel shank dimensions taken at minimum tolerance.

So you can get an "idea" of chamber pressures from PRE measurements.

If you study the charts closely you will see that the barrels do not like to experience more than 0.0019" of expansion. Expansion arount 0.001" to 0.0015" are what most factory loads generate. Looking at the pressures these expansions generate closely matches with the SAAMI standard pressures. Interesting?

ASS_CLOWN

Interesting post, AC. Could you possibly make the images a little bigger? Us ol' farts don't have the visual acuity we did in the old days, and this looks like useful stuff.



A simpler approach to whether PRE can work is the test for self consistency that I proposed. If a measurement method is not repeatable, nothing else about it matters much. My results say that the PRE method is about 40% information and about 60% random error, over a 30,000 PSI range. With that degree of non-repeatabiity, a large number of cartridges must be averaged to get acceptable precision.



If memory serves correctly, most factory ammo causes a temporary increase of about a .0005" in the circumference of a 1.2" barrel, but I'd have to check my notes to be sure.

Denton,

I will try and sharpen the images. This is a problem with taking pictures of print outs I am afraid.

You comments about the expansion of a 1.2" OD barrel is reasonable. It is effected by the chamber ID too.

The calculations I performed are taken in the clearance cut which is just forward (forward meaning muzzle end) of the barrel shank threads. Thus the importance of the thread size. This part of the barrel expands the most during firing as it is the thinnest / least well supported. Although the threaded portion will expand almost as much (and possibly as much depending upon the actual fit between the barrel and receiver threads)!

Again, just to clarify. The pressures I have provided in the aforementioned spreadsheets are not actual. They are a MINIMUM pressure you would have based upon the measured PRE you get. In reality they WILL ALMOST CERTAINLY be higher.

ASS_CLOWN

I love AC's posts, but also have trouble with the diagrams.
I save them on disc, look at them with Irfan View, and enlarge until my old eyes can see the little numbers.

Denton,

I sent them to you as an email. They should be far easier to read that way.

Clark, I will mail them to you too. I have to get out of here and don't have time to screw around with the photo editor.

ASS_ClOWN

Thanks!

Now the penny drops... your expansion figures were ID, which will grow more than OD does. Takes me a while, but I usually get there.