The ballistic pendulum
by B.B. Pelletier
Many of you have wondered how the gun makers of centuries past were able to test the power of their guns. This blog has touched on a few of the ways gun power was measured over the years, with the Splatology discussion being the most significant. If you are not aware of that report you really need to read it, because it’s the Rosetta stone that unlocks the mysteries of the past when it comes to airgun power.
Another way the shooters of history determined relative power was by the use of the ballistic pendulum. You can find a wealth of information about the ballistic pendulum online. Just do a Google search for ballistic pendulums and see what turns up.
The first big bore airgun match needed a ballistic pendulum
My own experience with ballistic pendulums dates back to the time I ran the first big bore airgun match at the August 1998 Mid-Atlantic Airgun show. I needed a way of determining the relative power level of the guns in that match and, even though we had and used modern electronic chronographs to record the power of the guns, I wanted something more–something that visibly showed the power so everybody could see it. I wanted what didn’t exist at the time–a ballistic pendulum for big bore airguns.
Gary Barnes was helping put the big bore match together, so he took on the project of building a ballistic pendulum for it, too. What he wound up building was a large spidery machine with a 4-inch steel plate the shooters had to hit. I wanted the match to be challenging, so we placed the pendulum 40 yards from the firing line. Believe me, in 1998, hitting a 4-inch target at 40 yards with a big-bore airgun was considered a big deal!
The ballistic pendulum was two feet high and four feet long.
A pen under spring pressure recorded how far the pendulum arm swung.
The steel plate was backed by more steel to make the pendulum arm heavy and to strengthen the plate.
Put up or shut up
In fact, hitting the target at all was the principal motivation for building the pendulum the way it was made. At the time, there were all sorts of stories circulating about super-powerful big bore guns with unbelievable accuracy. Among them, some smoothbore Farco air shotguns were supposed to be getting 2-inch 5-shot groups at 50 yards. Yeah–right! We ran this first-ever modern big-bore match as a “put up or shut up” affair, and not surprisingly a lot of shooters had to shut up.
At the first match, there were several guns that could not hit the target even one time in five shots. Out of 11 shooters, only 6 managed to hit the target even once out of five tries, and one of the six just scraped paint off the edge of the plate without recording any energy on target.
The score was determined by a combination of hits on the 4-inch plate and energy recorded by the pendulum. That last part was very arbitrary, as I will discuss in a moment, but back to the scoring. The more energy a gun had, the higher number of points were given for each hit on the plate. Powerful, accurate guns were rewarded, and weaker guns that were still accurate were penalized. The thinking behind that was that we wanted to reward power in combination with accuracy.
The lessons of the pendulum
How the first match turned out was less important than the lessons that the big pendulum taught us. First, it taught us that no rifle can ever expend all of its energy on a ballistic pendulum. How much it does expend is really how much that gets recorded, and that can only ever be a fraction of the total energy available. Let’s look at some energy thieves and learn why ballistic pendulums aren’t foolproof.
1. Loss of energy through projectile deformation
When a bullet hits a steel plate of any mass, it deforms, taking some of the impact energy with it. If the bullet hits at a very high speed, the deformation is more violent, robbing more energy. A lightweight bullet from a high-power centerfire rifle, for instance, will turn into lead powder and guilding-metal fragments, with an accompanying flash of light caused by the heat of impact that flashes some of the lead dust to incandescence. A slower, heavier projectile will not fragment as much and will impart greater energy to the target, causing the pendulum arm to swing farther. A ballistic pendulum constructed as this one was will show that a 400-grain bullet moving at 750 f.p.s. has greater energy than a 55-grain bullet moving at 3,200 f.p.s., even though the smaller, lighter bullet actually has more than twice the energy of the larger bullet.
This bias can be offset a little by selecting a different medium for the target. For example, if a big log is used and positioned to be hit on the end by both bullets, the smaller bullet will make a much better showing. That’s because the log will absorb more of the bullet’s impact energy without allowing as much deformation.
The pendulum has friction in several places. The arm that swings has bearings with friction, and the pen that records the energy also has some friction. Granted, these are both small forces, but they’re still real and they do matter. Barnes made the arm of the pendulum ride in a long bearing that was oiled and exercised frequently, but it still retarded the swing angle of the arm.
As the pendulum arm swings, it soon comes under the influence of gravity. The moment it swings past 90 degrees, gravity starts pulling at the swinging arm, slowing it down.
4. Glancing blows
When the bullet hits the plate, the plate begins to move. The bullet expends its energy by deforming, breaking into dust and flashing to incandescence, but it also glances away from the plate fairly fast. As it goes, it carries some energy with it. Other ballistic pendulums have been built with bullet (pellet) traps in their pendulum plates. The plates were shaped like funnels, so the bullet was deflected ever inward and continued to expend energy against the plate.
The energy thieves were but one lesson the pendulum taught us. Another was how arbitrary our measurements really were. If you look at the lines drawn on the scorecard, they’re supposed to represent foot-pounds of energy. Even when I made up the scorecard, I knew I was drawing the lines arbitrarily because I had no good way to calibrate them. Oh, I did shoot a few rounds with a blank scorecard in the machine so I could get a rough approximation of the energy needed to deflect the pen, but it was far from accurate or exact. It was never calibrated because I had no good understanding of how the energy thieves acted. Nothing was linear, either. As I mentioned earlier, a light, fast bullet was penalized, compared to a slow heavy bullet. The farther the pendulum swung, the more gravity acted upon it.
The next year, Ray and Hans Apelles showed up with single-shot Career 9mm rifles. We had added a 50-yard accuracy test to the ballistic pendulum test and they taught us another lesson. On the pendulum, they used a bullet that weighed over 175 grains, if I recall. I think it even weighed over 200 grains. But it was hell on the pendulum, with most shots smacking the plate sideways or nearly so. It did what they wanted it to do. But on the target at 50 yards, they used a 9mm pistol bullet that probably weighted 115-125 grains. Far more stable and accurate at airgun velocities.
Gary Barnes shot a .563 Express to win the match.
Bob Chilko shot his homemade .398 multi-pump pneumatic.
We learned another big lesson about big bore accuracy from the team of Bob Chilko and his son, Mike. They competed with their homemade smoothbore big bore guns. Bob shot an underlever and Mike shot a front-pumper that took 30 strokes to pressurize for every shot. Mike hit the 40-yard target four times in five shots and had everyone talking. How could he do that with a smoothbore? It turned out Mike, who was a physicist, had designed dumbbell-shaped slugs for both guns that had such high drag they flew accurately without a spin on them.
I noted that within the year, Gary Barnes was selling similar bullets for his big bores, and they pushed the distance at which his rifles were accurate from about 50 yards to 200! He called them Hornets, for the noise they made in flight. These bullets were a reincarnation of the French Balle Blondeau shotgun slug of the 1960s that revitalized the rifled slug industry.
So, the bottom line with the ballistic pendulum was that it provided everyone with an extended course in practical physics. In a world where accurate electronic chronographs abound, there isn’t much reason to have one of these. But at one time, they were the best that money could buy.