Pressure PointsBy John Barsness
©Copyright 2011, The Varmint Hunters Association, Inc.
Even today’s rather sophisticated handloaders often have questions about pressure, the force that makes rifle bullets go. Sometimes their questions even indicate some skepticism, partly because so much of today’s reloading data apparently disagrees. Why does one source list a maximum powder charge for a certain bullet weight that’s 1.5 grains above another source’s maximum?
The first step is to test the brass to see how the case wall resists pressure. This is done by pressure-injecting liquid into the case, and measuring the result.
Many veteran handloaders also note that maximum loads often have been reduced over the past few decades, something that’s generally attributed to lawyers. Often these veterans started handloading when loading manuals described the physical signs of excessive pressure, mostly in the appearance of fired cartridge cases. If the case was all right, they were told, pressures were all right. Nowadays they’re being told to never exceed published maximums — and many maximums seem pretty wimpy, especially when compared to the old days.
Long-time handloaders also have found that wildcat cartridges match or exceed the velocities of larger factory cartridges. If the .223 Remington Ackley Improved can match velocities of the .22-250 with less powder, why bother with the .22-250?
These anomalies tend to convince some handloaders that loading data is merely a suggestion, much like traffic laws in downtown New York City. Their only purpose is to guide beginners, not experienced veterans.
What’s the real story on pressure data? Much of the answer lies in the way it’s gathered, one reason I recently revisited the Western Powders pressure laboratory in Miles City, Montana. My first visit took place a decade ago, around the time Western (a long-time powder distributor) first started marketing their Ramshot line of powders.
It takes a lot of handloading equipment to run a pressure lab.
At that time the lab consisted of a single universal receiver and a few dozen piezo-electronic pressure barrels, and was run by one guy, the late Don Hartman. Today it’s grown into one of the top labs in the country, with three receivers and a staff of several technicians.
The Ramshot line of powders has also grown, and in addition Western acquired the Accurate line of powders. Between Ramshot and Accurate, Western now sells enough powder to be able to contract with manufacturers to make specific powders for their two lines. Accurate powders were formerly surplus powders, but these days they’re specifically manufactured for Western, with many improvements over the old line.
On this trip I spent an afternoon with Keith Anderson, one of the veteran technicians in the Western lab, testing the latest version of Accurate Magpro, a slow-burning powder. We decided to see what it would do with 100-grain Nosler Partitions in the 6mm Remington.
This article originally appeared in Varmint Hunter Magazine, and appears courtesy the Varmint Hunters Association, Inc. The VARMINT HUNTER Magazine is a popular publication of the VHA. Each year, the VHA 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”.
Essentially, modern pressure-testing is a more sophisticated and therefore sensitive version of the load “work-up” any handloader performs when putting together a new combination of powder and bullet. The big differences are that the laboratory technicians don’t have any previous data, but they do have pressure-testing equipment.
This bullet trap is where pressure-test bullets end up, a few feet from the muzzle. No, they’re not tested for accuracy.
Keith obviously didn’t know how well Magpro would work in the 6mm Remington, but he does have years of experience with various cartridges, bullets, and powders. He thought Magpro might be a little slow for our particular combination, but since we were in a pressure lab it was easy to do some shooting and find out.
Pressure isn’t tested merely shooting a cartridge and reading the pressure. The piezo-electronic machine that’s become pretty much the modern standard in North America uses a pressure barrel with a round hole in the chamber. A piston is inserted into the hole, one end of the piston conforming to the curve of the cartridge case, the other pressing on a transducer. When a cartridge is fired, the wall of the brass case expands and presses on the piston, sending an impulse to the transducer. The transducer sends this information to a computer. At the same time a chronograph reads the velocity. Many labs, including Western, use Oehler Model 57 chronographs.
The number that results, however, isn’t of much use unless the technician first figures out how his brass and pressure barrel differ from the standard setup used by the Small Arms and Ammunition Manufacturer’s Institute (SAAMI). First, one of the cases from the batch to be used in the test is placed in a separate machine that pressure-injects the case with liquid. This measures the toughness of the case wall, which obviously affects how much pressure is applied to the piston in the test barrel’s chamber. This case-wall toughness is the first “offset,” which is a number applied to correct the raw data obtained from the test firing.
All test barrels also vary slightly from each other, even when made by the finest custom barrel makers. A difference in bore diameter of 0.0001" (one ten-thousandths of an inch) results in a difference in chamber pressure of 800 to 1,000 pounds per square inch (psi). Tiny differences in the chamber dimensions also affect pressure.
These variations in bore are offset by first firing reference ammunition provided by SAAMI. As an example, let’s say this ammunition was tested in SAAMI’s standard barrel and found to produce 55,000 psi. If it produces 2 percent more in the Western lab’s barrel, then a 2 percent offset is subtracted from their raw data.
After Keith pressure-tested one of the Winchester-brand 6mm cases to be used, and some Remington reference ammo, he was ready to actually load some rounds. This was done in a separate room, but when I first visited the lab a decade ago the ammo was loaded in the same room.
Keith explained why this changed: “The SAAMI ammo is tested at sixty percent humidity. Miles City’s air is much drier than that, and we discovered our powder was drying out and raising pressures about four thousand pounds per square inch.” Now their test powder is stored in a humidified, temperature-controlled room.
Home | Next Page>