"Reprinted with permission of Shooting Times / November 1992"

Would you believe "real-world" ballistics at your shooting bench? And how about a revolutionary acoustic target system?

By Rick Jamison

Ken Oehler, the designer of the chronograph, has made a major breakthrough in ballistics measurement by introducing the Model 43 which is capable of measuring and calculating not only velocity, as most chronographs, but every significant aspect of exterior and interior ballistics.

With Oehler's System 43 a shooter with each pull of the trigger, has the capability of reading internal chamber pressures, instrumental velocity, muzzle velocity, ballistic coefficient (B.C.), point of impact, group size, wind deflection, downrange energy, time of flight, drop, and much more.

Not only does the new system display reliable numbers, and lots of them, it does it faster and easier than ever before possible. What's more, a pressure barrel isn't required; pressure testing can be conducted with most any sporting rifle, revolver, single-shot pistol, or shotgun.

I am enthusiastic about Oehler's new unit. It makes shooting more enjoyable- and more informative. For starters, you don't have to go downrange to change targets; you don't need a paper target, just a consistent aiming point.

You don't have to write any of the results down. They're all recorded automatically on a laptop computer. Not only are there tables of numbers in the results, a time/pressure curve is displayed for each shot. All this is stored in the computer's memory for recall at any time. The unit even remembers the sequence of the shots in the string. Fire 10 shots, and it will tell you which "bullet hole" was the fourth, second, etc. And once it's in memory it's saved without having to write it down or even print it out!

Portable computer sits atop Oehler M43: results of firing session can be printed out.

The velocity clocking and B.C. determination is so much easier to attain and record than what was previously possible with existing technology. The Oehler system is amazing and its potential is mind boggling. I think it will revolutionize ballistics knowledge and reporting. The new system has the potential to make handloading and shooting safer. No longer is a shooter "working in the dark" regarding pressures in his specific firearm. A shooter developing data for a wildcat cartridge has been on his own in the past, regarding maximum loads. The Oehler System 43 provides the means for an average shooter to develop much more reliable and safe load data.

One variation in displays of the M43 shows information added prior to a test. The data and firing results can be saved on disc for future reference.

To simplify things, I'll discuss each aspect of the new unit separately.

The Heart Of The Model 43

The Model 43. works in conjunction with a laptop computer. Actually, any computer; it's just that a laptop or notebook computer is more convenient to transport to the range and the Model 43 is about as transportable as conventional chronographs. The Model 43 is a flat silver box measuring only nine inches front to back, 11 ¼ inches wide, and 1 ½ inches thick. The top of the silver box serves as an ideal platform for the average laptop computer to rest upon.

The Model 43, working in conjunction with the computer, performs all the complex mathematical computations in response to the signals received from the pressure-measuring device, the chronograph screens, and the acoustic target frame.

To use, the shooter first enters information into the computer by filling in blanks on the screen regarding details about the firearm, ammunition, ambient conditions, and chronograph/target screen setup. When a shot is fired, the results are displayed in several forms. The chronograph/target screen setup is illustrated at the top left of the computer screen, complete with measurements. This diagram clearly illustrates whether the downrange setup is a target screen or a second chronograph (optional). The distances to each screen and/or acoustic target are also clearly indicated on the screen so that it's easy to check information entered into the computer.

At the upper right portion of the screen is a representation of group size and position. A group measurement (to the nearest .10 inch) is automatically provided with each shot. The last shot in the group is indicated by a blinking representation of the point of impact.

Internal chamber pressure is presented first in a diagram at the lower right portion of the screen which illustrates a time/pressure curve. It also illustrates muzzle exit with a "tick" mark on the curve.

In addition to these diagrams, a chart on the left side of the screen indicates instrumental velocity for each shot, this velocity discrepancy with the proof channel (if any), the time of flight, downrange velocity, B.C., horizontal and vertical shot dispersion, peak pressure, area under the pressure curve, and the pressure rise time.

There's more!

Another chart provides a summary including an average, standard deviation, high, low, and extreme spread both velocity and pressure figures.

If you wish, you can ask for another screen of information in chart form regarding downrange results from muzzle to 500 yards, with a built-in range default dependent on muzzle velocity centered on 2000 fps. If velocity is more than 2000 fps, a shooter can get information out to 500 yards. If below 2000 fps, the system defaults to 250 yards. Thus information includes bullet path, wind deflection, velocity, energy, and power factor.

This amazing system computes all these figures from the information derived from the pressure-measuring unit, chronograph. and acoustic target.

Pressure values can be determined by measuring the amount of compression a small billet of copper, called a crusher, receives during firing.

Pressure Measuring

The pressure-measuring unit on the Model 43 is revolutionary. It marks the first time that an individual shooter can get chamber pressure figures from his hunting rifle, shotgun, or handgun.

Those who conduct pressure measuring in elaborate pressure guns will be amazed at the ease, speed, low cost, and reliability of this portion of Oehler's new system.

I recently returned from a week-long school conducted by Ken Oehler, in conjunction with Bill Davis, Charlie Fagg, and Cliff Simonds. Davis and Fagg have extensive ballistics backgrounds, having worked for the military in this field.

During the school sessions, we compared pressure measurements with a copper crusher system, various types of transducer systems, and the Oehler Model 43. In one instance, we were able to derive simultaneous transducer and Oehler System 43 pressure readings on the same rifle. The uniformity and predictability of the Oehler system, when compared with industry-standard systems, is amazing.

I'll try to explain it as simply and clearly as possible.

The Oehler system utilizes a strain gauge attached to the outside of the firearm directly over the chamber area. The strain gauge measures the amount of strain the metal undergoes during firing. This stress is picked up in changing resistance in the tiny zigzag wire pattern on the gauge. These electrical impulses are represented in the form of a time/pressure curve, mentioned earlier.

The system relies upon the principle that metal, subject to a given level of stress, will change electrical resistance in the gauge a known amount, depending on the gauge, the dimensions of the metal, and the "modulus of elasticity" of the metal. The term «modulus of elasticity" is important because it tells how much stress (force) is required to induce a given strain (stretch) in the metal part. The term "yield strength" becomes important here. As long as the metal's yield strength is not exceeded, the metal will return to its original dimensions when the stress is removed. As long as the yield strength is not exceeded, the amount of resistance change relates directly to the pressure applied. In this instance, the type of steel is not a significant factor. It doesn't matter whether the barrel is made of conventional steel or stainless, the modulus of elasticity is constant and the results are predictable and valid.

Graphic illustration of screen/target setup shows group size,
peak pressure, muzzle exit, and velocity.

Unlocking The Pressure

The key in the pressure-measuring system is in the attachment of the strain gauge to the steel. In order for the system to work properly, the gauge must be firmly glued to the steel. The steel must be thoroughly cleaned in preparation for glue adhesion and a spot of blueing will likely be removed in the process. After glueing, it's best to cover the surface of the strain gauge and wire leads with epoxy to make them more durable.

The biggest drawback is that some shooters won't want a worn spot of blueing, a strain gauge, or a gob of epoxy on a prized rifle. However, after seeing the information to be gleaned from pressure measuring, I have no hesitation putting gauges on most of the rifles I own.

The ability to measure pressure, at least in a relative sense, is extremely valuable, and has the potential for lending a considerable margin of safety to handloading. A handloader now has the means to know about the pressures he's dealing with.

``You can learn more about real-world ballistics in a week by using this unit than you could in years of studying books."

Oehler recommends comparing pressure levels with factory ammunition first. I believe a handloader should not exceed the pressures generated with factory ammunition as measured in the same manner.

During the school, we had the opportunity to see the "fallacy" in the commonly used means of pressure determination. All it takes is a harder-than-normal lot of brass, and none of the conventional pressure signs (brass flow as evidenced by difficult bolt lift or extraction [a shiny spot on the base of the case], primer pocket expansion, or even measurable case expansion) show up at reasonable pressure levels. It's not all that uncommon for even proof loads to appear safe from these commonly relied upon indicators of pressure. In reality, the case has failed when brass begins to flow. The bandleader has already exceeded safe pressure levels. Oehler's system can change all that and you can learn a lot more about the uniformity of your loads in the process.

The knowledge to be derived by such a system is an education in itself. This element of the System 43 is phenomenal.

Strain Gauge Installation

Note wire direction of strain gauge, which allows revolver's cylinder to rotate.

The pressure readings from the Model 43 system are relative for that strain 9 gauge installation on that gun only. It's important that the user assume a certain degree of responsibility for the resulting pressure readings of the Model 43. It's imperative that the strain gauge be installed exactly as indicated in the instructions received with the unit. If the strain gauge is not installed properly, or if the wrong kind of glue is used, for example, it's possible to produce lower than actual pressure readings. For this reason, its important that the user not ignore; any of the classic indications of high pressures such as an excessively flattened or cratered primer, brass flow into the ejector hole of the boltface, difficult or sticky case extraction, etc. It's imperative that the user back off by 20 percent from the level when these signs originate and to check the installation of the strain gauge and connecting wires, along with the operation of the entire system.

Outside diameter is needed for computing pressures with the strain-gauge system.

Velocity Clocking

Again, Oehler has gone far beyond the normal velocity clocking that we're used to. Near the muzzle the Model 43 utilizes a conventional three-screen setup with the Oehler Proof Channel. The difference is a cable that connects to a second set of downrange screens or an acoustic target. Either can be used, depending on the setup you have.

Since the acoustic target is new, let's assume you're using it downrange. The cable connecting the near muzzle and the acoustic target allows time of flight clocking in addition to the target coordinates. From these figures, B.C. and target velocity can be derived. B.C. is a measure of how streamlined a bullet is. This is indicated by how much velocity is lost during flight over a specified distance.

Author and Ken Oehler make sure the cable connection to the triangular acoustic frame is properly attached so the microphones in each corner can accurately record each shot.

The beauty of the acoustic target is that it allows you a huge window to shoot through downrange. This has been a problem for me in the past when it comes to downrange bullet velocity clocking, which is necessary to derive a B.C. It's difficult to get a downrange clocking on every shot and it's easy to shoot a screen downrange when a variety of bullet styles and loads are being fired. The new acoustic target frame changes all that.

Unlike conventional chronograph screens which are triggered by light-sensitive cells, the acoustic target screen is triggered by the shock wave of the passing bullet. For this reason, it's important that the bullet velocity exceed the speed of sound (1117 fps at sea level) at the downrange screen.

Acoustic Target

Acoustic targets have been used by major ballistics labs, but until Oehler's System 43, the acoustics haven't been affordable or available to the average shooter.

Basically, the acoustic target is a triangular frame of plastic pipe. The size of the frame can be adjusted but the smaller the frame the greater the accuracy. A frame 62 inches on a side is what we used during the school. This size is capable of determining bullet position within .20 inch as it passes through the frame.

There is a special microphone at each corner of the triangle. Each microphone picks up the shock wave of the bullet when it passes. The microphone nearest the bullet picks up the shock first, the second nearest, second, and the farthest microphone picks it up last. All this takes place over a tiny fraction of a second but there is a longer time delay the farther the microphone is from the bullet when it passes.

By means of sophisticated electronics and triangulation, this system determines where the bullet is in space when it passes through the frame. What this means, amazingly, is that a shooter doesn't need a target. All he needs is an aiming point. He can shoot all day long without going downrange or using a spotting scope. As he fires, group size and shot position is indicated on the computer screen, as mentioned earlier.

By looking at the frame with microphones, it's difficult to believe the accuracy of the unit. But I can attest that it works perfectly.

One person in the group fired a five-shot string at 100 yards on a target posted immediately behind the acoustic frame. While the shooter was downrange retrieving his target, Ken Oehler plotted his own target on a piece, graph paper to illustrate the accuracy of the unit. Ken placed an X where the computer indicated each bullet had passed through the frame. When the shooter returned with his target, Ken placed the target with bullet holes over the System 43--predicted Xs on the graph paper. Amazingly, the holes aligned perfectly; there was an X at the center of each bullet hole. It made a believer out of me!

Ken Oehler's new Model 43 Personal Ballistics Laboratory is properly named. You can learn more about real-word ballistics in a week by using this unit than you could in years of studying books.

The Beauty of The Oehler M43

The beauty of the Model 43 is that now there are rules to the game of working up loads, making the process much safer. In the past, a wildcatter could get practically any velocity he wanted as long as his cases would contain the pressures.

Sometimes, harder than normal brass would allow very high pressures without failing. According to Oehler, and with the use of the Model 43, no one should be working higher than a 60,000 indicated pressure with the Oehler unit.

Those handloaders who thought they were improving the pressure/velocity performance of factory loads will find that they have their work cut out for them. A lot of time and effort has gone into the development of factory ammunition, and those who thought they could equal its performance with handloads previously haven't had the means to know what their loads were doing pressure wise. I'm referring to peak pressure, pressure rise time, area under the pressure curve, and pressure uniformity.

Again, the Model 43 applies rules to the game, making it immensely safer and more enjoyable due to the added information the system provides.

In a recent phone conversation with Ken Oehler, I learned that the official readings from the Model 43 be referred to as "psi (M-43)."