Long Range Hunting Online Magazine

Pressures, Case Strength and Back Thrust
A popular varmint cartridge of the day is the .223. It is designed for a maximum product pressure of 55,200 c.u.p., a little hotter than the .30-06, but the case web measures only about .192" thick. This might seem a little inconsistent when compared to the lower pressure and thicker web of the .30-06, but the area inside the .223 case is much smaller than inside the .30-06 and this results in less real pressure working back against the bolt of the .223 in the form of back thrust. The inside chamber pressure is high, however, and therefore the .223 case must be kept to less than .192" unsupported protrusion out of the barrel for safety.

Pressures, Case Strength and Back Thrust
(L-R): .270 Winchester; .30-30 WCF; .22-250 Remington; 6mm BR Norma. The web thicknesses are .202", .182", .218" and .162", respectively.


When I cut into the .300 Weatherby belted case, I expected to find a very thick, substantial web. It turned out to be only a little thicker than the .30-30 and the same as the small .223, measuring .192" thick. Next, I sectioned a .264 Winchester case, made by Winchester, and found it to measure .215" thick at the web and about .200" near the flash hole. It is interesting to find that belted magnum brass may not be any thicker at the web than .30-06 brass, and the chamber pressure of the .300 Weatherby is listed in various places as something around 57,000 c.u.p., or about 7% above that of the .30-06.

A favorite wildcatter’s brass is the .284 Winchester rebated case. The web turned out to measure slightly thinner than that of the .30-06 at about .200" thick. Since it is a rimless case, headspacing in the same manner as any other rimless case, it would present the same potential problems with one other added in. That is the fact that it is of the rebated rim type where the rim is the same diameter as that of a .30-06, but the body is much larger starting just ahead of the extractor groove. This will create a larger internal head area which leads to higher back thrust on the bolt when compared to smaller diameter cases with the same chamber pressure. The chamber pressure for the .284 is listed at 57,200 c.u.p., about the same as the .300 Weatherby and the .264 Winchester.

The next case I used for this investigation was the popular benchrest case, 7mm BR, originating from a straight bodied .308 and later from a .300 Savage case. This case is commonly necked down to .25, 6mm or .22 for use in benchrest rifles where pressures are habitually run up considerably above 55,000 c.u.p. The web turned out to be much thinner. My measurements showed the thickest part measuring about .196" and the thin part around the primer pocket only .180" or so in Remington brass. The Norma BR measures only .162" web thickness, and the Lapua BR is .168".

Pressures, Case Strength and Back Thrust
The case on the left is a .270 formed into the 6.5x06 Ackley Improved. Notice the stretch valley just in front of the solid head. The .22-250 case on the right shows no stretching, no thin stretch valley near the web.


These case web measurements are not too informative when we consider that all have been used extensively for wildcatting and all have been successful even with unknown pressures of more than 60,000 c.u.p. This seems to lead to the conclusion that the main concern might well be the method of chambering each wildcat cartridge. If the cartridge is well contained inside the chamber with only a minimum cut for the extractor, it will work well with very high pressures.

Back thrust, mentioned above, brings us to another consideration for cartridge cases used in experimentation or wildcatting. Even though we read of the chamber pressures being in the 50,000 c.u.p. range in most high powered cartridges, it has been proven that back thrust on the bolt is retarded by the fact that the case wall expands to grip the chamber wall, thus holding the case from backing up hard against the bolt. This also indicates that the case head will hold a great deal of pressure by itself, and this is where the web thickness becomes very important. If the gap between the bolt face and the barrel breech is wider than the thickness of the case web, the high chamber pressure inside the case will blow through the thin sidewall in the unsupported gap area. And while we all hear of rifles blowing up, that is not the case. It is rather the loose pressure blowing out from an unsealed chamber when a case failure occurs. As long as the 50,000 c.u.p. plus can be retained inside the chamber, receiver ring and barrel, it is rare that a failure occurs. Steel will contain much more than 50,000 psi, and tests on actions and barrels have proven they will hold more pressure than will the brass case that seals the pressure off. Actions have been tested far beyond 75,000 psi with no failures. The brass, however, will begin to flow at something over 70,000 psi, and will totally fail by the time the pressure reaches 80,000 psi. Then the release of all that pressure inside the receiver, down through the magazine, back through the firing pin hole and into the bolt body is enough to split the stock into kindling wood...among other things. I have the remains of a Mauser M98 action that was totally destroyed with a standard loaded .22-250 cartridge when the headspace became too long, allowing the case to separate. The brass cartridge head was welded into the ejector slot in the locking lug and part of the case body at the end of the web area expanded and formed between the bolt face and the butt of the barrel in a tight manner, looking a lot like it had been melted and poured into the gap. I had to remove the barrel in order to open the action. Incidentally, the shooter ended up in the hospital emergency room for removal of metal and carbon fragments, his eyes being saved by the glasses he was wearing. All of this was brought about by the failure of the brass case when the soft M98 locking lug seats finally pounded back far enough to make the headspace too long. Perhaps I should add here that Mauser actions are not heat treated like our modern factory actions. They are made from relatively soft carbon steel and then only surface hardened, case hardened, for a very thin, hard surface. When these M98s are reconditioned many shops will lap the locking lugs and often will cut the thin, hard surface completely away, leaving only the soft carbon steel underneath to hold the pounding of the bolt locking lugs in the future. They will pound back over a period of time resulting in too much headspace and a wrecked rifle...or worse.

The amount of back thrust on a bolt can be changed to some degree by the design of the cartridge case. Cartridges with considerable taper in the body will produce much more back thrust than will a more straight walled case. To prove this, P.O. Ackley tested the very tapered .250-3000 cartridge in a lever action Savage M99 rifle by loading it up until the case became difficult to extract. This occurred with a load of 34.5 grs. of powder and a 100 gr. bullet to produce a velocity of 2900 fps. At 36 grs. of powder, the action became frozen and required the use of a rod to pound down the barrel while pulling hard on the operating lever to get it open. He then rechambered the rifle for the almost straight tapered case of the .250-3000 Ackley Improved cartridge and loaded it with 36 grs. of powder and a 100 gr. bullet, the very load that previously froze the action. Upon firing, it extracted very easily. He then continued to increase the load until reaching 42 grs. of powder for a velocity of 3400 fps. The action still opened easily. Thus, he had removed most of the back thrust that had originally frozen the action with the tapered, standard case and was able to radically increase the velocity, along with the pressure, without an increase in back thrust by simply removing most of the taper from the case. Today, most experimenters use cases with very little taper for their wildcat creations.

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