The Powley Computer & PSI Calculator
January 04, 2011
Using Homer Powley's ballistic equations, Bob Forker designed the Powley Computer and psi Calculator.
If you go by the letters I've received asking about the Powley Computer for Handloaders and the complementing Powley psi Calculator that I have occasionally mentioned in my columns, there are at least a few more old codgers like me out there who handload and remember these early reloading tools. ST reader Wayne Reveen from Alaska and others have written and asked, "Where can I buy them?" The short answer is, "I don't know."
Homer Powley and the Hutton Rifle Ranch used to advertise these slide-rule type devices for sale in various shooting publications. However, I haven't seen the ads for many years, and both gentlemen are deceased. I bought mine 30 years ago for less than $10, and only recently did I acquire another set from fellow InterMedia Outdoors writer Bob Forker. Forker has written about reloading in our sister publication, Guns & Ammo, for most of the past 40 years, and coincidentally, I recently learned that he helped develop the Powley calcualtion tools. I wasn't really surprised because I had seen him in photographs that illustrated Robert Hutton's "Technically Speaking" and "On the Technical Side" columns in G&A during the mid-1960s.
So I called Forker to discuss the readers' inquiries and finally asked why he hadn't written a column about the origins of the Powley Computer. He promptly replied, "Well, I'd feel a bit self-conscious about tooting my own horn." Then I suggested that I write about it, but only if he agreed to help me tell the whole story.
The mathematics required to analytically predict the internal ballistics exhibited by a cartridge when it's discharged was published in German technical documents nearly a century ago. We can only surmise that U.S. military and commercial munitions makers knew all about this as well, but they didn't share it outside their intimate circles. Remember, this was the time when the transition to factory ammo loaded with smokeless instead of blackpowder was almost complete. Because of the intrinsic hazards of loading the then-new high-energy propellants versus the comparatively benign blackpowder universally used for centuries, handloaders were a rare breed. Whatever technical knowledge that existed then was not readily available to John Q. Citizen. Besides, only a few individuals were interested anyway.
The factories did not encourage handloading because the propellant technology was so different, and besides, they could surely make more money selling loaded cartridges instead of the raw components to shooters. It wasn't until after the Great Depression and World War II ended that the renewed interest in shooting sports and handloading began.
Most of the "experts" who wrote about handloading in the 1940s and '50s typically did not have access to laboratories or precise test equipment, so they couldn't precisely measure ballistic performance. They developed handload recipes the old-fashioned way--by trial and error. I remember reading a writer who declared the most effective way he determined the maximum load for a specific bullet/propellant combination was to incrementally increase the charge weight until his rifle exhibited "heavy bolt lift," and then he backed off a grain or two on the charge. Obviously, neither Shooting Times nor I advocate this technique. Back then, there were only a couple of loading manuals--Lyman and Belding & Mull--available for many years after the war, and the handloader's circumstances didn't significantly improve until the 1960s.
Before there were PCs, DVD-Rs, and digital software, we used analog devices called slide rules and other tools such as graph paper and pencils to perform serious figuring and plot data.
The Powley Computer was first advertised in Shooting Times in the February 1963 issue (top). In that same issue, ST ran a product announcement of the new handloader's tool.
In 1962, Forker worked for a company that designed slide charts, mostly for product advertising. Powley approached Forker's employer to see if they could adapt his ballistic equations for loading DuPont's single-base, nitrocellulose IMR propellants into a slide-rule computer. Talk about good fortune!
Because Forker knew a little something about guns, he got the job. He said, "When I first saw Homer's work, it was as if someone just turned on the lights. Here was a rational way to safely determine load recipes and predict cartridge performance using a few readily defined factors, e.g., bullet caliber and weight, barrel length, and case volume."
Powley engaged in voluminous typed correspondence, including sending carbon copies (pre-Xerox era) of his missives to interested parties. Forker noticed that one recipient, Robert Hutton, lived nearby and asked Powley if he could contact him to get more insight into the project. That was the beginning of his ongoing relationship with reloading and writing about it for G&A.
Forker recalled, "I could count higher than 10 without having to take my shoes off, so Hutton recruited me into the G&A loop." Forker had soon designed the computer scales and layout and actually prepared the slide-rule artwork. The first production run of 3,000 units sold out quickly, and numerous reruns followed until many years later when the various PC-based digital ballistic models pushed Powley's less sophisticated analog scheme aside.
Hutton and Powley teamed up to write an article titled "Pressure Estimation by Chronograph," which appeared in the March 1963 issue of G&A, and after it was published, Hutton encouraged Forker to put Powley's latest pressure-analysis equations into another slide rule. Soon the Powley psi Calculator was a reality. It is not a stand-alone tool; it complements the Powley Computer. Using both, you can determine a safe handload and predict its velocity. First, you verified the Powley prediction by actually measuring the velocity of the test load fired in your rifle. Then, the psi calculator accurately estimated the load's peak pressure.
Here's how the two tools work. During my recent loading project with the .308 Norma Magnum, I chronographed velocities and recorded figures ranging from 2,800 to 3,000 fps for several different 180-grain bullets propelled by various charges of multiple propellants from the rifle's 24-inch barrel. Using the Powley Computer, the .308 NM's case capacity of 81.5 grains of water (to the base of the neck) indicates 70 grains of a to-be-determined IMR propellant as the appropriate charge weight. So the ratio of charge to bullet weight is 0.39. Aligning the bullet's sectional density (0.272 as listed on the back of the slide rule) with the 0.39 value determined previously, the slide points to "4831" on the powder number scale.
To estimate the load's velocity, the slide is adjusted so the .30-caliber hash mark on the lower scale is aligned with the barrel length to the base of the bullet (approximately 22 inches). The expansion ratio corresponding to a case capacity of 81.5 grains of water reads almost exactly "6."
The calculator is then turned over, and the slide is adjusted so the ratio of charge to bullet weight pointer is set on 0.39, and under the number 6 in the upper expansion ratio window you can see the estimated velocity of 2,840 fps.
Switching over to the psi calculator, the upper slide is adjusted to align the grains of powder (70) with the case capacity (81.5). Holding it in place, the lower slide is adjusted so the ratio of charge to bullet weight (0.39) is aligned with the expansion ratio (6). Then the pressure at the velocity (actual or predicted) hash mark is read. It reads 45,500 psi. Look's like the velocity prediction is pretty close, but why is the predicted pressure so low?
IMR propellants, for which the Powley Computer was designed, remain the standard to which the performance of today's myriad new powders are often compared.
Remember, back then pressure was measured using the copper crusher method. Powley and Hutton referred to the measured values as pounds per square inch, but it actually wasn't. Today, copper crusher values are called copper units of pressure (CUP). Unfortunately, there's no reliable method to accurately correlate psi and CUP, but that's a topic the ballistics editor can expound upon better than I. The test rifle's chamber had more than a quarter-inch of freebore so "real" pressures of my handloads were probably in the 55,000 psi range, which is perfectly safe for a modern bolt-action rifle in good condition.
Because the original Powley tools were set up for IMR powders, Hutton later prepared a chart to indicate potentially suitable alternate propellants, i.e., those exhibiting comparable burn rates to the various IMR types. I've used the chart to correlate the proposed load derived by the Powley Computer with other types of propellant such as Hodgdon spherical or Alliant double-base powders.
However, let me emphasize a serious caution: There are as many different orders of propellants based on indicated burn rate as there are burn-rate charts. You can't simply substitute the same charge of a non-IMR propellant indicated as having the same or similar burn rate as the corresponding IMR powder. You must confirm the recommendation with other reputable sources, and if it still looks promising, start with at least a 5-percent reduced charge and work up, if feasible.
So, that's the whole Powley story. I'm sorry the answer to Mr. Reveen's question remains the same. I don't know where you can get the Powley Computer today.