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Pressures, Case Strength and Back Thrust
Another interesting test by Ackley was with the .30-30 Ackley Improved cartridge in a M94 lever action Winchester. He found a dry chamber would allow the cartridge case to firmly adhere to the chamber walls; the rifle could be fired with the locking lugs completely removed from the action and only the lever left to hold the bolt shut. The straight walled case did not back out of the chamber, proving that back thrust had been almost completely eliminated by the cartridge case design.

Pressures, Case Strength and Back Thrust

Back thrust is normally figured by multiplying the chamber pressure by the internal area of the cartridge head. It does not equate to chamber pressure as most shooters might think. If it did, then the back thrust of the .223 and the .460 Weatherby would be the same because both are loaded to a chamber pressure of about the same, 55,000 c.u.p. In reality, the .223 has a back thrust of only about 4,025 pounds, while the big .460 has 10,425 pounds, even though the chamber pressures are about the same. Thus, it is readily seen that when given a choice we would much rather have the 4,025 pound load on the bolt face, right near our head, than the 10,425. And this is exactly why we should consider the case we plan to use for our latest wildcat design.

In any chamber the only two parts of a cartridge that are not in some contact with the chamber are the primer and the bullet. The bullet has the barrel opening directly ahead of it, so it will move forward rather easily upon firing the round. The primer is held in the head of the cartridge, in the primer pocket, with only a little friction to secure it. When the cartridge is fired the full pressure developed within the cartridge case will be pushing forward on the bullet base and backward on the case head creating what we call back thrust. Back thrust through the small .080" flash hole has been calculated to be in the 600-800 psi range, and of course the bolt face should be snug against the case head. The primer should be seated slightly below the case head and the initial pressure through the flash hole will push the primer back into solid contact with the bolt face. The pressure inside the case will also work against the case wall to expand it out against the chamber wall. This will cause the case to adhere to the chamber very tightly, preventing the case body from moving back as much as will the case head. Naturally, this results in stretching the brass case slightly and can lead to case head separation. And now, because the bolt and barrel do not make positive contact, it is readily seen that the brass cartridge case head is retaining at least some pressure to prevent it from blowing out in the barrel-bolt gap. Since force is a factor of effective area times pressure, by knowing the area of the inside of the case head we can calculate an approximate back thrust pressure for each cartridge. I say approximate because it would be difficult, if not impossible, to determine back thrust restriction caused by the cartridge case form and adherence to the chamber wall. In other words, back thrust is contained by a combination of the bolt locking lugs and the case clinging to the chamber wall. Ackley's experimentation along these lines proved that a properly designed case will hold back a surprising amount of back thrust from the bolt and locking system of a rifle, but only when the chamber is kept dry. By oiling the chamber or cartridge case before firing, back thrust will be increased tremendously, and may even approach that of full chamber pressure.

Below are examples of back thrust calculations for three cartridge head diameters with equal chamber pressures of 50,000 c.u.p. The chamber wall friction resulting from case configuration is ignored for ease of calculation, and these back pressures are considered to be maximum (see table below).

From this chart it is easy to see that if you plan to use a belted magnum case for your wildcat cartridge, it will produce more than 20% additional back thrust when compared to using a .30-06 case loaded to the same 50,000 c.u.p. chamber pressure. Another way of stating it is that you could increase the load in the .30-06 case to a chamber pressure of 60,000 c.u.p. and still have less back thrust on your bolt than if using the magnum case at only 50,000 c.u.p. And this is one reason why it is so often fruitless to use the belted case for your brain child, lightning striking wildcat....

In recent years the ammunition companies have been avoiding the use of belted cartridge cases for new magnum designs. They have moved to the smooth bodies, no belts, of variations of the old .404 Jeffery case, headspaced on the shoulder. The case rims are rebated slightly so as to fit standard belted mag bolt faces. Since the inside diameter of the case head is somewhat larger than that of most belted mag cases, the back thrust will be increased to somewhere around 9,000 pounds. But since these cases are manufactured to better tolerances than are belted cases, I would use the non-belted case for wildcatting, rather than the belted model, even though the back thrust would be greater. Our rifles handle back thrust better than poor headspace….

At one time I was a great fan of the .264 Winchester Magnum. It used a bullet diameter of .264, or 6.5mm, which I thought would be a great combination bullet for varmints and big game. The factory listed the 100 gr. bullet at a velocity of 3700 fps. I had used a .257 Weatherby earlier, only to discover that it would not meet the early day advertised velocity of 3720 fps with a 100 gr. bullet, and even the factory finally conceded that point, dropping the advertised velocity steadily downward to the 3550 fps of today. Therefore, I had great hope for the .264 being much better. That was back when chronographs were coming on the market, and I had just received one from my wife for Christmas.... Don't all wives give that sort of gift to their husbands...? Naturally, the chronograph told stories that were greatly different than what appeared in rifle and cartridge advertisements. The first box of factory .264 ammo with 100 gr. bullets checked out at only 3186 fps! That about blew my mind. I started reloading the cases with loads that would beat that by several hundred feet per second. By the time the velocities reached 3500 fps, all sorts of bolt lift problems, primer pocket expansion, and case head expansion, etc., developed. With a top load of 69 grs. of powder and a 100 gr. Sierra bullet, the velocity was 3525, but the case was expanding .002" on the belt and .005" just ahead on the body. If the case was new, the primer would not leak, but on the second or third loading, leaks showed up. Of course, I had to pound on the bolt to open or close it. By keeping the velocity down to about 3450 fps, things were pretty good, and that became my top usable load. With the 120 gr. bullet I found the top load to be just a bit over 3200 fps with relatively short case life. These loads all produced pressures close to the maximum of 57,200 c.u.p., based on the condition of the cases and primers.

I liked that .264, but not with the problems. My research seemed to indicate that I could build a rifle and use the .30-06 case necked down to .264 and probably get the same, or better, velocities. If so, the back thrust on the bolt would be relieved, too, so I ordered a reamer for the 6.5x06 Ackley Improved, a regular wildcat. To my happy surprise, the 6.5x06 Ackley gives the same top velocities as did the .264, but without case problems and with five or six grains less powder. Had I been more into case design before obtaining the .264 Mag, I might have figured out in advance that the 6.5x06 Ackley would have been the better choice. Isn't it amazing how good hindsight is...?

Pressures, Case Strength and Back Thrust

When you finally decide to start playing around with gun building and cartridge designing, remember to keep your tolerances close, use the smallest case head diameter that will give the performance you are after, and keep all that chamber pressure under control. And keep shooting.

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