Long Range Hunting Online Magazine

Pressure Points
Only new brass is used in testing, since fired brass is work-hardened and produces different results. Keith first pushed the neck of each case over the expander ball in a sizing die. “This is to standardize bullet pull, which can also affect pressure. If it’s not done the variation from round to round is much larger.” I remarked that I do the same thing with new brass when handloading, and for the same reason.

Keith nodded vigorously, but a few minutes later, when seating the bullets he noted, “I’m feeling a little variation in how much leverage it takes to seat each bullet. That probably means there’s some variation in neck thickness, but we don’t neck-turn test brass.”

He started with a load that definitely would be under maximum, but filled the case approximately 90 percent full. Over the years Keith has become a big believer in full cases. Full cases usually exhibit less pressure and velocity variation, resulting in finer accuracy, especially at longer ranges. “The ideal is a powder that fills the case to the base of the bullet, or is just slightly compressed, not more than five percent.”

It took less than five minutes to fire the 10 test rounds. That’s the reality of a pressure lab: 95 percent of the time is spent involved in getting ready to shoot, not in shooting.

Pressure Points
Reference ammunition is used to determine how the pressure lab's test barrel differs from SAAMI's standard barrel.

The result was an average pressure in the lower 50,000 psi range. Keith nodded in satisfaction, then did a little figuring on a hand calculator. “I think this might work out to be a good load. Part of it’s the Nosler Partition bullet. Partitions tend to produce more pressure than most other bullets of the same weight. If the bullet developed less pressure, we’d probably run out of room in the case, but I think we’re going to get a case full of powder at just the right pressure.”

“What’s the bullet that produces the least pressure?” I asked. “I’ve noticed Hornady Interlocks usually produce less pressure.”

“Barnes Triple-Shocks normally produce even less,” Keith said, “especially the lightweight ones with the fewest driving bands. Often I have to go to a slightly faster-burning powder with light TSXs.”

It used to be almost an article of faith among many handloaders that the best accuracy came from loads somewhat under maximum. This was true of some of the powders most commonly used a few decades ago, such as IMR 4895, originally designed for .30-06 military ammunition used in the Garand semiautomatic. The Garand functioned best at pressures around 50,000 psi, the level where IMR 4895 burns most consistently.

Of course, part of the problem may also have been that most older data was worked up in the same way many handloaders work up loads, by adding more powder until the case or rifle starts showing signs of distress, whether expanded primer pockets or hard bolt lift. The load was then backed off a grain or two.

Modern piezo equipment, however, has shown that loads worked up with the “distress method” often result in pressures that are way too high. A perfect example is the 7mm STW, originally a wildcat cartridge. Many people who shot 7mm STW rifles back then reported muzzle velocities of 3,500 to 3,600 fps with 140-grain bullets, with no signs of excessive pressure. When Remington decided to turn the 7mm STW into a factory cartridge, however, they pressure-tested those fast loads and found most were in the 70,000 psi range, and some even higher.

Pressure Points
Keith Anderson could feel a noticeable variation in the pressure needed to seat the test bullets, which can result in extra variation.

What would be the problem with that? Isn’t a load safe as long as the brass holds up? Well, no. The modern accepted maximum pressure level of about 65,000 psi is the highest pressure modern steels can withstand over and over again. If you keep feeding even a very strong bolt rifle 70,000 psi ammunition, some part of the action is likely to fail. If there’s a flaw somewhere inside the action, say in one of the locking lugs, it will fail even sooner. Modern rifles are built with considerable margin of error, but aren’t infallible.

In fact, there’s a dent in the sheet-steel wall on the outside of the Western Powders lab, a testament to the unwillingness of a (former) employee to believe in this basic fact of handloading safety. He designed a wildcat varmint cartridge that supposedly developed a lot more velocity than other cases with similar powder capacity.

Since the cases held up for several firings, he assumed pressures were OK, until one day when he was sighting-in his rifle at the outdoor range next to the pressure lab. The rifle’s action came apart. Luckily the shooter wasn’t hurt, but the action’s disintegration was so violent that it blew the scope off the rifle, flinging it into the side of the building more than 20 feet away — where the dent remains.

On the other hand, modern powders designed for bolt-action rifles burn most consistently at around 58,000 to 65,000 psi. Until that pressure level is approached, there’s typically wider shot-to-shot variation in pressure, and hence accuracy. While some handloaders tend to ignore listed maximums in the search for ever more velocity, many other handloaders still believe in the sub-maximum rule of accuracy. They quit testing before their loads approach the listed maximum, assuming the powder isn’t really suited to their cartridge/bullet combination. If they just added another grain or two, however, the powder would burn more consistently, and accuracy would improve. I’ve seen this over and over again, and so has Keith Anderson.

Pressure Points
The actual shooting doesn't take long. Ten rounds typically will be touched off within less than five minutes.

Keith did some figuring on a hand calculator to decide what powder charge to try next, instead of just increasing the charge a grain at a time. (This is fairly standard procedure when you’re testing loads in a very strong universal receiver, but don’t try it at home.) We then went back to the loading room where Keith put together 10 more test rounds with the increased powder charge.

Again, loading the rounds took more time than shooting them, but when Keith looked at the numbers on the computer he nodded in satisfaction. The average pressure was just under 60,000 psi, and average muzzle velocity just over 3,100 fps. “The standard deviation is a little higher than I’d like,” Keith said, pointing to the computer screen. “But that’s probably due to the variation I felt in the case necks. This is a good load, and will be in the next batch of Accurate data.”

It was close to quitting time, and since it was the first sunny day of the year, after a very wet spring, we went outside to sit in lawn chairs under the dent in the steel wall, and discussed pressure a little more. I asked why Western didn’t use the copper-crusher method of pressure-testing, since I knew they tried it at first. Was there a problem with accurate readings?

“Well, you can get accurate readings with copper crushers,” Keith said. “But you saw how much time we spend pre-measuring the offsets. With copper-crusher testing, there are a lot more measurements. They not only take more time, but leave more opportunity for error.”

We eventually started talking about the so-called “pressure signs” of home handloading. “Since I started getting some ammo tested here,” I said, “I’ve noticed that many traditional warning signs, like loose primer pockets, don’t show up until pressures are over sixty-five thousand psi. So in my articles I tell readers to watch the chronograph, and when they start seeing velocities above maximum, then pressures are above maximum too. Just like they are when you test.”

“Oh, yeah,” Keith said, smiling and nodding. “A chronograph is the handloader’s best friend!”

The VARMINT HUNTER Magazine, a 208-page publication put together for shooters by shooters. The Varmint Hunters Association, Inc. hosts several 600-yard IBS matches, a coyote calling contest, and an annual Jamboree in Fort Pierre, SD. The Jamboree is a week-long shooting event known as "a summer camp for shooters".

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