Bullet Ballistics: Velocity
Measuring gas pressure proved as difficult at first as measuring bullet velocity. Then, in the mid-1800s, Alfred Nobel and an American named Rodman came up with solutions to that problem at the same time. Rodman’s, the crusher system, is still in use.
It’s a factory procedure not easily or safely performed in a home shop. A small cylindrical piston is slid into a hole in the barrel of a test gun, and a copper or lead pellet is inserted snugly between the top of the piston and a stationary anvil. When the rifle is fired, the piston pushes against the pellet or crusher, shortening it.
The difference in crusher length before and after firing is then converted mathematically to a pressure range, in units of CUP or LUP (copper units of pressure or lead units of pressure).
Copper crushers are generally either .146 in diameter and .400 long to start with, or .225 in diameter and .500 long. Choice depends on application. Copper crushers work best in centerfire rifles and handguns that generate substantial pressures. Lead crushers (.325 x .500) typically register the low-pressure loads in rimfire guns and shotguns (though small-diameter copper crushers can be used too). Crushers are calibrated in a test press.
Pounded by high pressures, crushers don’t register peak pressure accurately because the flow of copper is slower than the change of pressure in the chamber. Also, the moving piston must be brought to a halt, which skews a reading in the opposite direction.
Bullet Ballistics: CUP
Copper units of pressure (CUP) and lead units of pressure are not the same; nor can they be interchanged with another common unit of pressure, pounds per square inch (PSI).
A CUP value may coincide with a PSI value; for example, SAAMI lists 28,000 as maximum average pressure for the .45-70. Both CUP and PSI units apply. But maximum average pressure for the .243 is 52,000 CUP and 60,000 PSI. Most cartridges show similar discrepancies. Sadly, there’s no easy way to convert CUP to PSI or vice versa.
A modern device for pressure measurement in firearms is the piezoelectric gauge. It registers an electric charge delivered through a transducer when a crystal is crushed. Pressure applied to the crystal yields a proportional transducer reading in pounds per square inch.
Conformal transducers are installed in the barrel, just like crusher pistons, and become part of the barrel. External transducers can be mounted on the barrel, then removed for replacement or calibration checks.
Another pressure tester that’s become popular among shooters is the strain gauge. Developed for consumers by chronograph guru Ken Oehler, it’s essentially a length of wire you glue to the outside of the chamber wall. When you fire, the chamber expands and the wire stretches. That stretch translates into pressure. It does not equate with readings from a crusher or a piezoelectric gauge.
Learn More About Bullet Ballistics
Bullet ballistics information is like a rabbit hole. The more you learn, the more you want to learn. Dive deep into this subject in the latest edition of Cartridges of the World. It contains more than 1,500 entries on popular and rare ammunition.
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About the Author: Wayne van Zwoll is a regular contributor to the Gun Digest annual, and author of the Gun Digest Book of Sporting optics. He is a nationally-recognized expert on rifles, optics and western hunting.
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