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Pressures, Case Strength and Back Thrust

Pressures, Case Strength and Back Thrust

By Bob Jourdan
©Copyright 2010, Precision Shooting Magazine

All handloading manuals take safety very seriously. They cover most of the hazardous points in reloading ammunition in an effort to keep a beginner out of trouble. Some go into detail concerning the strength of brass cases and the importance of using the proper case for the proper loads. They also have some discussion of chamber pressures, advising reloaders what to watch for as they approach top loads that could result in problems. This is all usually done in a good manner, but of course it does not get into deep, technical detail, nor should it. These books are written for reloaders, experienced or beginners, and often refer the readers to other more technical books and manuals for greater detail on the subject. As we all know, any average intelligent shooter can become a proficient ammunition reloader and enjoy the hobby for a lifetime. But as with any consuming hobby, the quest for more knowledge and advancement comes all too soon. Continued success with common factory cartridges often leads the curious minded to start looking at wildcats or improved cartridges, and even the development of some new, personal design that will send off their favorite bullets at the speed of light...or faster. At this point the shooter must investigate the possibilities of case failure and the consequences of excessive chamber pressure.

Pressures, Case Strength and Back Thrust
Two extremes, the .300 Weatherby Magnum, (L), and the old-time .30-30 WCF, (R). Notice the thickness of the case web.

One of the most important developments in firearms history was the brass cartridge case that first came onto the scene just before the Civil War. In 1856 a dentist named Maynard patented a brass cartridge case that would expand when fired and seal the rifle chamber to close off any escape of powder gases. This forced all pressure to push behind the bullet and send it out the barrel with the best velocity. Sealing of the chamber was a major step in the development of breech loading rifles. Prior to that, all breech loaders spit a lot of fire and brimstone out the rear of the action into the shooter's face.... They were not any too popular in those days.

With time, shooters wanted more velocity which meant more pressure. When smokeless powders came along in 1886, first in France and then all around the world, pressures began a steady move up. By the turn of the century they had reached the 50,000 c.u.p. (copper units of pressure) that is common today. This copper unit of pressure is simply a description of how chamber pressure came to be gauged by use of a crusher gauge, invented by Alford Noble in about 1860. Small carefully controlled cylinders of soft metal, copper, called “crushers” are placed in a tool attached to the chamber area of a rifle barrel where a hole has been bored through the chamber wall and side wall of a cartridge case. When the cartridge is fired, the high pressure gases push a small piston in the tool up against the copper cylinder, compressing it to some degree. The copper crusher is then measured and the amount of crush length is compared to a compression table to arrive at the pounds of pressure required for that amount of copper compression. Then we use the term, copper units of pressure to relate it to the shooting world.

Pressures, Case Strength and Back Thrust
Here we have the .22-250 Remington, (L), and the .30-30, (R). The .22-250 has a two-thickness web, .218" at the outer interior diameter and .195" in the center area about equal in diameter to the primer pocket.

Even though we still use the c.u.p. term for chamber pressures in many publications, there is now an improved system giving pressures in pounds per square inch (psi). It uses electronics to arrive at pressures and is called an electronic-transducer gauge. Instead of crushing a copper piece, a special quartz piece is used because quartz has the property of generating an electrical charge when subjected to mechanical force. This is now known as a piezoelectric transducer, and many loading manuals now list pressures as psi. As reloaders, we must be aware of the significant pressure differences between c.u.p. and psi, with the psi always being much higher. An example would be a .30-06, 150 gr. bullet at 3005 fps for 45,900 c.u.p. or about 58,000 psi. This pressure difference of 12,100 pounds requires your attention. Don’t mix c.u.p. with psi.

Good rifle design became very important to contain this pressure, since all chamber seal was provided by the relatively thin brass case. And brass simply can't handle pressures as high as steel can. From this, shooters can see why it is commonly stated that the weak point of any rifle is the brass cartridge case. In a properly designed rifle, the cartridge case will protrude out of the barrel at the breech by about 1/8" before reaching the bolt face. In bad set-ups the distance can be as much as 1/4". With an open gap like that it is easy to see that 50,000 c.u.p. inside the thin brass case can blow it out and release the pressure in the action and shooter's face. Thus, any gunsmith knows, or should know, that the deeper the cartridge is seated into the full cover of the barrel, the safer will be the rifle. Naturally, there must be a way to hook an extractor over the rim of a cartridge to withdraw it from the chamber, and this is where the difference in case head protrusion has to be considered. A large and poorly designed extractor will require the case head to protrude more than would a small, well designed extractor. And in some rifles an extractor cut is required in the barrel. If this cut is made too deep, the cut will expose the thin sidewall of the cartridge case and set up a possible blowout.

It is often stated that a certain case is built stronger than another. In many instances this is true. But in others, it is not. Usually the original cartridge will be designed to handle the pressure of that one particular load. For example, the original .30-30 Winchester cartridge case is designed for pressures in the range of 40,000 c.u.p. This means that the brass is sufficiently thick to handle that amount of pressure in original .30-30 rifle designs. By carefully sectioning the head of a .30-30 case the solid web thickness can be measured and compared to the web thickness of other case designs. My recent measurement showed the .30-30 web to be approximately .182" thick, which means that anytime a cut is made in the barrel breech that exposes more than .182" of the case, the thin wall of the case is all that will be left to hold that 40,000 c.u.p. If a gun designer made a cut of .200" or so to make room for the extractor in the barrel breech and used the .30-30 cartridge case, a disaster would be in the making....

Since the .30-30 was reputed to be a rather weak case, I decided to section a few others to compare their web thicknesses. The .30-06 case showed a web thickness of two dimensions in a Remington case. The center, approximately the diameter of the primer pocket, measured only about .190" thick, while the remainder of the web measured .202". Why they thinned the center around the flash hole, I do not know. Nevertheless, this indicates that any case protrusion of more than .200" from the barrel should be considered dangerous. The maximum product pressure for the .30-06 is listed as 53,200 c.u.p., and that would be exciting if it got loose in the action of your favorite rifle. Since this is a rimless case it is common for the case head to protrude from the barrel breech by some amount, and a few rifles do have a cut made in the barrel to let in an extractor.

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