Firearm Chamber Pressure Testing FAQs

There are a lot of questions about firearm chamber pressure testing; here are some of the most common ones with some answers.

July 27, 2021 By Allan Jones

In a column I wrote almost 12 years ago, I discussed the differences between copper crusher testing and piezo-electric transducer testing and how that affects the data we ballisticians publish. I still get questions about pressure testing and load development. Here are some common ones.

One common question I receive goes like this. How is a pressure barrel different from a regular barrel? The answer is a pressure/velocity (p/v) barrel built to precise industry standards does not fit any existing hand- or shoulder-fired firearm. Unless severely modified, it fits only a Universal Receiver, a massive chunk of precision machining with delusions of being a crew-served cannon. The receiver bristles with ancillary supports for sensors and a heavy breeching/firing mechanism, and barrels are ported for those sensors.

Barrels for a Universal Receiver are massive. Their breech end is 2 inches in diameter and 4 inches long to perfectly fit the special test receiver. The unsupported portion is 1.250 inches outside diameter for the full 24-inch length (for most rifle cartridges).

In creating reloading data, we employed the same standardized equipment and protocols used to develop and monitor the production of factory small-arms ammunition. I was paying about $700 per barrel in 2005. If setting up for transducer testing, we had the additional expense of the piezo-electric transducer and a transducer calibration fixture, running the total to about $2,100 for a rifle cartridge.

My techs loved Krieger’s single-cryo ASTM 4150 chrome-moly barrels because normal jacket fouling accumulated more slowly, reducing both time and aggressiveness required to properly clean the bores at the end of a shift. In my business, one always strived to keep his techs happy and his pressure barrels healthy!

Reference Ammunition

I often mention “reference ammunition” in my Shooting Times column, and readers often follow up, wanting to know what that is. Reference ammo is calibration ammo. For any standardized test system to safely serve an entire industry, it must allow each member company to calibrate its test equipment to a common standard.

That common standard in ballistics testing is reference ammunition, usually production ammunition selected from one loading lot that demonstrated good uniformity. It is thoroughly evaluated by other SAAMI companies’ test stations and barrels. Each station reports its observed pressure and velocity values for the lot to SAAMI’s Technical Committee, who, if the data profile is acceptable, assigns assessed values to that ammo lot. The company that made the ammo uses special packaging and separate inventories for the use of other member companies.

Calibration works like this. Let’s say a reference lot’s assessed values are 2,975 fps and 57,595 psi. We fire that lot in our p/v barrel, and it posts 2,950 fps and 58,240 psi. We know to add 25 fps to our velocity readings and subtract 645 psi from our pressure readings to match the barrels in other companies’ stations. Those are “correction factors.”

Reference ammo also helps monitor barrel condition and service life. Pressure barrels wear with use, so each reference lot has “exclusion limits.” If you must enter correction factors larger than those exclusion limits, you need to carefully inspect that barrel for fouling. If that is not the issue, you buy a new one.

That discussion inevitably brings up a question about “secondary reference.” For a variety of reasons, official reference ammo is sometimes unavailable, requiring secondary reference. Either my most senior tech or I worked directly with the cartridge’s developer. That company would reevaluate a sample lot for pressure for the purpose of our order.

We would not use these assessed values as SAAMI correction factors but rather use the pressure reading we obtained in our barrel as a “do not exceed” point. This kept our recommended loads within the pressures to which the factory loaded. We found that worked best when we ordered the heaviest bullet weight offered in that cartridge.

Equipment Durability

And one of the most common questions is: Is a heavy pressure barrel indestructible? Close but not quite. Fortunately, the pressure sensors can act as sacrificial blow-off vents during any serious “pressure excursion,” often saving the barrel. A bent crusher piston wasn’t too bad; our machine shop always kept spare blank pistons on hand. They could have one fitted to the barrel and back to us in a day or so for minimal cost.

I don’t know how many times over the years I’ve been asked if data development is time-consuming, but that one has been asked a lot. The answer is very much so. In addition to the technician’s time for loading and firing, I had to develop a load plan. When shooting was done, I analyzed the data and assigned safe start and max charge levels for every combination of cartridge, propellant, and bullet. Time is money, and so data development is costly.

In estimating “shoot times” for transducer testing, I usually figured 40 man-hours for one bullet in one cartridge with 12 to 16 propellants. If we had to shoot on crusher, I cringed a little and added 50 percent to that estimate.

Starting Points

The last question I’ll present here is people often want to know how we knew where to start testing with a new cartridge. I have been accused of keeping a wizard’s wand in my office, but we relied on more traditional means. Really. We used modified software to model a cartridge to identify propellants likely to perform best. This avoided “fishing expeditions” that increased barrel wear. The goal was to set a range of testable charge weights that delivered safe, publishable data with minimal reshoots. The predictive software never ever replaced live-fire testing.

However, the best tool was experience. The techs I was blessed to have had between 40 and 50 years of reloading experience each. They knew propellants and which cartridges might need special attention. In the long run, it comes down to having great people. I did.