|
Rocky’s Reloading Room |
|
Expansion of fired cases is measured just ahead of the web, at the maximum case diameter. A high quality micrometer and a practiced hand are required. |
|
Measuring (?) Pressure
Many times I’ve been asked how I determine how much pressure a given load develops. The answer is that I don’t. That is, I don’t determine the actual pressure in pounds per square inch. What I can observe, however is relative pressure, and that is enough.
Let me explain: The actual pressure that a given rifle or pistol load develops when it is fired is important, both for performance reasons and for safety. As reloaders, we are vitally concerned with both; but we lack the means to measure pressure. The big ammunition companies have laboratory equipment that costs well into the millions of dollars, and even it only gives a rough approximation of what happens inside a cartridge case when the powder burns. That equipment can, however, determine peak or maximum pressures of ammunition. We amateurs have no such systems available to us. (Yes, I know all about Dr. Ken Oehler’s Model 43 Personal Ballistics Laboratory. It is truly a magnificent system for the very advanced amateur, but even he doesn’t claim that it measures actual pressures.)
The good news is that we don’t need million dollar laboratory systems that tell us actual pressures. Once peak pressures have been determined by a laboratory, all we have to determine is if our loads develop less or more pressure than factory ammunition. That’s what I mean by relative pressure.
Before I delve into this any more, allow me to give credit to a real expert. All that I’m about to explain was revealed to me by Ken Waters, a true reloader’s reloader. It is explained in much more detail in the first chapter of Ken’s classic “Pet Loads Volume I.” I urge you to study Ken’s words in great detail, as he is a masterful wordsmith in addition to his skills at the reloading bench. And now, back to my effort.
The pressure generated by burning gunpowder exerts force in all directions equally. Part of that force is used to accelerate the bullet down the barrel, and the rest causes the brass case to expand in the gun’s chamber until it is stopped by the chamber walls. That is a good thing, because the expanded brass acts as a gasket or seal. Thus, the 50,000 pounds per square inch or so of incredibly hot gas and incandescent particles are kept within the gun and not blown at hypersonic speeds into our tender anatomy. (Keep the image that has just formed in your mind very fresh, and recall it when you are tempted to add just one more grain of powder.)
The brass case, having now expanded as far as it ever will, holds its new size and shape – except for a small amount of “springback” – after the bullet exits the muzzle and internal pressure drops once again to ambient, or zero. Thinner parts of the case expand more, but also spring back more, than thicker parts. |
|
Where the case walls are almost their thickest, just ahead of the internal part of the case called the web, at its rear, the case expands to its largest diameter, and springs back least. The higher the pressure, the farther the case expands. And that, readers, is the secret of the method.
Let us take the venerable .30-06 case as an example. Using a precision micrometer that reads to four decimal places (.0000”), and measuring an unfired factory round, we find that the case measures .4700” in diameter at a point just ahead of the internal web (or about 3/8” ahead of the rim). After that round is fired, we measure again at the same point and find that it has expanded to .4720”. In THAT GUN, therefore, factory ammo expands .002” at what we must presume is normal working pressure for that caliber.
Now we prepare a reload using the same bullet weight and a powder charge at the start level from a current reloading manual. (We measure to be sure that our sizing die reduces the case to near-original size, as well.) After firing that round, we find that the case has now expanded to .4715”. We obviously have a safe load that develops less than factory pressures. A chronograph |
|
would reveal that velocity is below that of the factory load, as well.
If we elect to increase the powder charge in small increments, we might notice a progression of expansion measurements. Succeeding loads might result in diameters of .4717, .4718 and .4719”. The next identical increase in powder charge might produce a diameter of .4721”. The chronograph, if used, would reveal steadily increasing velocities until that of the factory load were equaled or slightly bettered. At this point, we can assume that the pressure being developed is roughly equal to that of the factory ammunition, and we have reached a maximum load for that rifle. That is where we stop, even if the loading manual shows higher loads.
If we were to proceed with even higher charge levels, we would note several things: cases would expand to beyond that of factory ammunition, velocities would show ever-smaller increases per charge increase, and we would begin to see other signs of excess pressure. Namely, we might now observe sticky extraction, cratered or flattened primers, bright spots on the head of the case, or other classic signs. But all of those classic signs only begin to appear AFTER we have already gone beyond factory maximum pressures!
Notice that we do not now, nor ever did, know the actual pressure of any load fired. The precise numbers are immaterial. If we can assume that the factory ammunition produces pressures that are no higher than established maximums for that caliber – and we may safely make that assumption – then we can also conclude that our own reloads do not exceed that same level of pressure.
Better yet, we can observe a working maximum charge BEFORE any other signs of over-pressure even begin to appear. It is this ability to observe the APPROACH to maximum pressure that is so valuable. All other methods of judging pressure reveal only that a safe maximum has been PASSED. Determining relative pressure by the case measurement method allows us to observe when we are below, roughly equal to, or in excess of the pressure of factory-loaded ammunition.
When using this method, there are several things that must be remembered: 1. You must begin with fresh factory ammunition using the same bullet weight as you intend to reload. 2. The results are only valid in one particular gun. If you reload for more than one gun in a given caliber, the entire procedure must be repeated for each separate gun. 3. You should use a single lot of brass cases, or the factory cases themselves, for the tests. Cases of different composition, hardness, or dimensions will invalidate the results. 4. You must use a precision micrometer capable of measuring to at least four decimal places, and also develop a consistent measuring technique. 5. You must measure only at the “expansion ring,” a slightly bright ring that appears on the case wall ahead of the internal web, roughly 3/8” from the rim or ¼” ahead of the extractor groove or belt. This ring is easily seen on any fired case, and is not to be confused with an even brighter ring that presages a case separation. 6. All other safety practices normal to reloading must be scrupulously followed.
For a much more detailed explanation of this method, I once again encourage you to refer to Ken Water’s original work in his Pet Loads Volume I.
Rocky Raab |