Life Expectancy: Factors That Affect Cartridge Case Life
August 13, 2012
The useful life of a cartridge case usually ends with an excessively expanded primer pocket or a split neck, and several factors can determine the number of firings it will withstand before calling it quits.
When a case reaches the final step in its forming process at the factory, work-hardening has made the brass quite hard over its entire length. The head of the case is left that way for strength, but its neck and shoulder are annealed to make them softer and more elastic.
If you bend a wire coat hanger back and forth repeatedly, it will eventually break at that point because it has lost its elasticity. That's work-hardening. The same thing happens when a cartridge is fired and then reloaded: Its neck expands away from the bullet, and sizing it back to its original diameter during the reloading process causes it to become harder — just like the coat hanger.
Expand and resize the neck often enough and it will eventually lose its elasticity and split. Softening the necks of cases by annealing as required will prevent this from happening.
A case with greater hardness in its head area will withstand higher chamber pressures before its primer pocket expands than will a case that's softer in that area. Manufacturers of commercial ammunition have never agreed on exactly how hard the head of a case should be, and there are no better examples of this than 6.5-284 Norma brass made by Norma and Lapua. A maximum load that will give long life in a Lapua case will expand the primer pocket beyond use in a Norma case in three to five firings.
The web and wall thicknesses of a case also have a bearing on the number of times it can be reloaded at maximum chamber pressures. The .22 Hornet and .218 Bee are similar in performance, but when both are loaded to the same pressure level, Bee brass lasts longer because it is beefier all around.
An easy way to extend case life is to load 100 fps or so below maximum velocity for a particular cartridge. The .22 Hornet is capable of 2,900 fps with a 40-grain bullet, but cases last much longer in the 2,700 fps range with very little sacrifice in trajectory. A long case life is more important to the high-volume varmint shooter than to the big game hunter who shoots only a few rounds each year.
Some reloading manuals tell us to screw a full-length resizing die into the press until it makes contact with the shell holder, but cases will last longer if the die is adjusted to barely bump the shoulder of the case back enough for trouble-free chambering.
Back the die out, size a case and try chambering it in the rifle. If resistance is felt when closing the bolt, screw in the die in quarter-revolution increments until the resistance disappears. Then tighten the lock ring on the die and it is set to resize cases for that particular rifle.
Since it work-hardens only the neck of a case, a neck-sizing die extends case life beyond what is possibly with a full-length resizer. I often use one when loading for varmint and target shooting, but I always full-length resize when loading ammunition for a big game hunt.
A case loaded to maximum chamber pressure and fired in a rifle with a rear-locking action — the Savage Model 99 lever action and the various models of Shultz & Larsen, for example — won't last for as many firings as one fired in a rifle with its locking lugs located up front. When a cartridge is fired in a rifle with rear lock-up, its wall grips the chamber wall, but slight compression of the bolt allows the unsupported head of the case to move to the rear. Repeated resizing and firing of the case can eventually cause it to separate just forward of its web in only a few firings.
Primers can also affect case life. Corrosive primers begin to attack brass and weaken it the instant the round is fired, so longevity was increased by leaps and bounds when Remington introduced the non-corrosive primer during the late 1920s. While commercial ammunition has had non-corrosive primers since then, ammunition loaded for the military in the U.S. and other countries continued to be loaded with corrosive primers for many years thereafter — and some of that ammunition is still being sold on the surplus market.
I am also sure some case so-primed are being reloaded. It is safe to do fairly soon after the ammunition is fired, but no one knows how long it takes for a case to become too weak to be safely reloaded. I know from experience that it will eventually happen.
During the 1980s I started competing in handgun metallic silhouette matches with a gun chambered to 7mm TCU. I shot thousands of rounds each year and formed cases from whatever .223 Remington/5.56mm brass I could come up with, including once-fired, military stuff. I was aware of the use of corrosive primers in military ammunition but had assumed the practice had been discontinued prior to the adoption of the 5.56mm by the U.S. Army in 1957. I was wrong.
I continued to load and shoot those cases for several years before I stopped shooting handgun silhouette. A few weeks prior to writing this column I decided to dust off the gun and ammo to punch some paper, and when I tried to pull a round from its 50-round storage box, the end of the cartridge containing the bullet broke off at the shoulder. Same thing happened on the next round. And the next.
All the cases were head-stamped "LC 84," which meant the cartridges were loaded at the U.S. Army's Lake City Arsenal in 1984. I have no idea what would have happened had I fired one of those cartridges, but my guess is it would not have been pleasant.
So beyond following accepted reloading practice, it pays to know where your brass came from and what it was loaded with. That certainly goes for those who scrounge range brass in military calibers such as .223/5.56 and .308/7.62.