June 02, 2021
Sometimes I am asked by serious handloaders and novice reloaders, “How do you read chamber pressure?” My short answer is, “You can’t, and no device yet designed directly does, either.” Let me explain.
I recently read some articles about how handloaders could “read” pressures. One thoroughly explored assessing chamber pressure by precisely measuring case-head expansion. The author spent big bucks on a digital micrometer that could measure 0.00001 inch! He fired hundreds of rounds of carefully assembled handloads and took about 1,500 measurements. After three pages of text, seven photos, and two charts, he summed up the results by stating, “…based on my experience, I don’t trust it to tell me much.”
Another article I read was a reprint from years ago that expanded the scope of reading pressures by describing how examining, measuring, and evaluating all the physical evidence exhibited by a fired cartridge case can allow you to “read” pressures. It summarized the contents of many other articles I’ve read about this topic, including how, for example, flat, cratered and/or pierced primers were clearly reliable pressure indicators. Those two articles reaffirmed my opinion that it’s best for handloaders to stay vigilant and make sure your handloads are safe to fire before firing them.
How do you do that? Well, you use proven load data published by reputable sources, and you use a chronograph to confirm that you are achieving velocities similar to the velocities those reliable sources got.
When I began rolling my own ammo 50 years ago, what I knew about chamber pressure was essentially nil. I had limited discretionary funds then, so saving money and being able to shoot more were good enough reasons for me to handload. Fortunately, I met two savvy handloaders who pointed me in the right direction. For quite a while, I loaded and fired hundreds of rounds without experiencing the consequences of making a stupid mistake.
Being an engineer, I soon looked beyond my reloading bench and gathered this and that piece of technical information related to my hobby. My first two reloading manuals were Lyman’s 45th and Speer’s 9th editions. Although naively unaware of any need to “read” pressures, I faithfully learned and followed the reloading recommendations in those load manuals. I also kept good records. Based on the targets, I knew which load recipes were accurate and which ones were not worth repeating.
One of my reloading mentors was John Redmon. At some point in my growth as a handloader, I concluded I should be “reading” pressures in order to be absolutely safe. I couldn’t afford a chronograph at the time, so I started looking at primers and attempted to measure case-head expansion. I usually couldn’t make heads nor tails of my observations.
Redmon convinced me the most important factor related to handloads (other than safety) is how accurately the bullets impact the target. After I acquired my first chronograph, I offered several times to fire his handloads and measure their velocities, but he never accepted my offer and repeatedly reminded me that he religiously followed the information offered in the reputable load manuals.
Velocity and pressure are directly proportional factors. In other words, velocity is a reliable indicator of whether or not your handloads are safe. Measuring the velocities of your handloads is easily done with any of the many available chronographs on the market. And fortunately, many of them are very reasonably priced.
You may ask, “What about the so-called hobbyist pressure-measuring devices available to handloaders?” Many years ago, the industry mechanically measured chamber pressures in LUPs or CUPs. Today “improved” electronic digital methods are the norm, and the measured values are usually reported in psi. However, these devices are externally mounted on the barrel outside the chamber and don’t directly measure chamber pressure. The most sophisticated hobbyist pressure-measuring devices employ strain gauges cemented onto the barrel over the chamber.
Let me be clear. You can make conclusions about pressure from the condition of the fired case. A black smudge around the primer pocket, smeared ejector impression, a loose or a missing primer, or, heaven forbid, seeing a tendril of smoke trickling out of the action are clearly signs that pressures are way too high. But you shouldn’t depend on the physical characteristics of your fired brass to assess pressure. It’s not reliable unless you’ve already experienced an unsafe and potentially dangerous condition.
However, you should look for recommended load data in load manuals that provide lab-tested pressure data and keep your handloads in line with it. You’ll likely find recipes for the actual components you’re loading or for a bullet of similar construction. The Hodgdon, Lyman, and Western reloading manuals include laboratory-tested pressure data for thousands of handloads. You can compare the data in them with other reputable sources of safe handload recipes.
Obviously, you shouldn’t purposely exceed safe pressures. You can avoid doing that by simply following Mr. Redmon’s advice.