October 25, 2018
Throat Erosion Effects
Barrels die because the area in the bore where the rifling begins, called the “throat,” erodes from the effects of heat and pressure. As the distance between a bullet that is seated in the chamber and the rifling grows, it becomes harder and harder for the bullet to remain straight prior to engaging the rifling. Once it starts to yaw or twist in the bore prior to hitting the rifling, accuracy quickly goes downhill.
Throat erosion also allows an ever-increasing amount of gas to blow past the bullet before it seals the bore when it hits the rifling. This is why barrels that are dying slowly lose velocity first. One of the surest indicators a barrel is entering its golden years is the loss of velocity compared to when it was young with about 200 rounds through the bore.
Cartridge selection is the single greatest contributor to how long a barrel lives. Some cartridges torch barrels quickly because of the high heat and great pressure they generate. With all that heat and pressure usually comes performance, so cartridges that are hard on barrels aren’t necessarily a bad choice. If you want a rifle that will deliver lots of energy at great distance, expect to replace the barrel sooner than later.
The issue is predicting how short barrel life will be. The best way to judge any cartridge and how hard it is on a barrel is to look at maximum SAAMI pressure, case capacity and bore diameter.
The key ingredient here is high pressure, barrels don’t last as the pressure rises. There is no getting around this principle. Pressurized gas in the chamber blasts the throat. The throat, under extreme heat and pressure, rapidly moves between the solid and liquid phase. The bore melts a little bit from the heat and the pressure blasts the metal away.
Most magnum cartridges have a maximum SAAMI pressure between 62,000 and 65,000 pounds per square inch (psi). Cartridges that operate at these high pressures will have a shorter barrel life than a .223 Remington and its maximum pressure of 55,000 psi. Factory ammunition is loaded with SAAMI maximum pressure in mind, so a cartridge rated to 65,000 psi will almost always have a chamber pressure higher than a cartridge SAAMI rated to 55,000 psi, regardless of the factory ammunition chosen. It pays to know SAAMI maximum pressure for any cartridge of interest.
As much as we all like velocity, increased velocity always means increased pressure, so barrel life drops. Handloaders have the unique option of finding what load a rifle shoots accurately. Often there is more than one powder charge that creates the best accuracy. If you want longer barrel life, choose the lower-velocity node. There is very little real-world performance yield from a 75 feet-per-second (fps) velocity increase, but a significant decrease in barrel life from the higher pressure.
In addition to SAAMI maximum pressure, the reloader must consider the cartridge’s case capacity. The powder in the case is the fuel that burns when the cartridge fires. More fuel means more heat. Heat is the second great enemy to barrel life and it’s just as egregious as pressure.
A large case capacity can be a sign of short barrel life, except when it’s teamed up with a large bore diameter. This is where a few benchmarks are useful.
The U.S. Army’s Special Forces Sniper Course (SFSC) states that barrel life for a .300 Win. Mag.loaded with a 190-grain bullet (with about 67 grains of powder behind it) is 1,200 rounds. Dave Emary, one of the lead designers of the 6.5 PRC while he was at Hornady — now a Guns & Ammo contributor — said that the barrel life of the 6.5 PRC loaded with a 140-grain bullet with about 55 grains of powder is 1,400 to 1,600 rounds. My personal experience with the 6.5 Creedmoor using a 140-grain bullet and 41 grains of powder sees barrel life hovering around 3,000 rounds. The table in this article puts these figures in a more presentable order.
A closer comparison of the 6.5 Creedmoor and the 6.5 PRC is illuminating. The Creedmoor has a maximum SAAMI pressure of 62,000 psi and the PRC has a max pressure of 65,000 psi. Putting an additional 14 grains of powder behind the same bullet and bumping pressure up another 2,000 psi cuts barrel life in half.
There are ways to mitigate the effects of heat and pressure if you just have to have magnum pressures and magnum velocity. Jimmie Sloan, the inventor of the .338 Norma Mag. and .300 Norma Mag. found a way to get double the life of a .300 Win. Mag. while throwing an additional 13 grains of powder down a .30-caliber hole. He designed both the Norma cartridges to slightly “Ackley-ize” during firing.
The .300 Norma Mag. has a chamber that is sized .0012-inch shorter than the cartridge length at the neck/shoulder junction while having twice as much room as the .300 Win. Mag. when looking at shoulder width. This means the .300 Norma Mag. cartridge gets held fast at the neck/shoulder junction when chambered while allowing considerable expansion of the shoulder width when the cartridge fires.
By keeping the pressure in the case busy reforming the shoulder during firing, Sloan found a way to redirect the pressure that would normally eat the throat. This slight delay in maximum pressure allows the bullet to get into the rifling and helps preserve throat life. As far as I know, this is the first time anyone has done that with a factory cartridge.
The amount of powder in a case is important, but the type of powder is more important. You would think a single-base powder (i.e., the so-called “cooler” powders) has a longer throat life than a double-base powder because the nitroglycerin in the double-base powder increases the flame temperature.
The Australian Department of Defense did an exhaustive study on the subject and the data shows that is not the case. However, the Aussies found that the chemical reaction between the powder and the barrel steel was a bigger driver of throat erosion than the flame temperature associated with the powder type.
There is no current rating system of what chemical composition yields the slowest throat erosion but I wish powder companies would explore this area. After temperature stability, this would be one of the most important aspects of powder selection for load development.
For the rest of us, the best way to manage throat erosion is to shoot as heavy a barrel as possible (the first 4 inches of it, anyway) and to let it cool as often as you.
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