by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
- How powerful were antique big bores?
- How were they filled?
- No one knew
- Hand pumps of antiquity
- Empirical testing
- Early rapper
- Single-stage pump tradeoff
- But wait — there’s more!
Dennis Quackenbush has always been helpful when it comes to the difficult questions about airguns. Over the years, he and I have experimented with several fundamental questions; the most recent being the $100 PCP. I should have an update on that one for you in a couple weeks.
How powerful were antique big bores?
Back in the 1990s — the days when I was still writing The Airgun Letter and Airgun Revue magazine — I had a prolonged discussion with Dennis about the performance of big bore airguns of antiquity. He had just come out with the .375-caliber Brigand that was about to start the airgun world on its modern journey toward big bores, and there was a lot of interest in them.
This report is not about big bore power, but the hand pumps that were used to fill them. I did write a blog about the power of big bore of the past that can be seen here.
How were they filled?
I wondered how powerful those old airguns were, and also how they were filled with compressed air. On Monday of this week I published the report on the air cane and one of our readers wondered the same thing. How could anyone fill an antique big bore airgun with air compressed high enough to launch a lead bullet with lethal force? There were no scuba tanks and no shoebox compressors back in those days (1600s-1900s). Indeed, there weren’t many devices that could compress air at all, to say nothing of the relatively high levels that were required by an airgun!
They didn’t even call them air compressors back in the 1600s. They were called condensing syringes, and compressed air was called condensed air. The terminology had yet to be established.
No one knew
It seems strange today with all that we know about big bore airguns, but in 1997 almost nobody knew how fast these old guns shot or what sort of pressure they used. I read all sorts of guesstimates of velocities up to 1,000 f.p.s. for the antique big bores.
Eventually splatology (the determination of terminal velocity by the shape of a deformed lead ball after hitting a hard surface) was developed by Gary Barnes, but Barnes came to the party in the late 1990s, and these questions were being asked several years earlier.
Hand pumps of antiquity
One thing we did know about the vintage big bores, though, is what their hand pumps looked like. Many of them survived along with the guns, and they were usually in better condition than the guns themselves.
I had examined antique hand pumps several times at airgun shows. They always operated very smoothly, like their pistons had been lapped. I saw the insides of a couple pumps and neither of them had a piston seal. They had a flat iron piston head on the end of the pump rod. A light to medium weight oil (like 3 in 1) was squirted into the pump before it was put into operation. Oil on the compression chamber walls did all the sealing — similar to how it works in an automobile engine.
I have read several accounts of hand pumps that have leather piston seals. They were either a single leather seal covering the end of the piston, or they were a stack of leather washers fastened together at the end of the piston. These presumably work the same way as the plain metal pistons, plus the leather helps keep oil on the walls of the compression chamber. I only know about the stacked washer type from reading Air Guns by Eldon Wolff. He admits that the leather washer type is much rarer than the plain piston type. But how effective were they? What sort of pressure could they generate?
I asked Dennis how much pressure a common single stage hand pump could generate. He’s been making replica hand pumps for vintage big bore guns for several years. Dennis’ pumps are true to the old designs, except that they use synthetic pump seals. He suggested that we test some pumps and find out.
Before we conducted the tests, it seemed to both of us that neither vintage pump design (plain piston or one with a leather seal) could equal one with a modern synthetic seal. Whatever pressure could be achieved with a modern replica would probably represent the absolute maximum for any vintage pump of the same physical specifications. That turned out to be an incorrect assumption.
We both agreed that the practical limit to the force that could be applied had to be the weight of the person doing the pumping. These early pumps had no mechanical advantage beyond that which is inherent in a single-stage mechanism. Although it would be possible to generate more force than one’s weight by pulling the base of the pump toward oneself, it isn’t practical — and would be very hard to do on a continuing basis. Jumping on the pump handle, however, is actually a method that was used sometimes. It was called rapping, but it’s very hard on the hands, wrists and arm joints and you can’t do it very long.
Single-stage pump tradeoff
Okay — remember when Mr. Conners, your 8th-grade math teacher, told you that one day you would need to know geometry? Today is that day. A single-stage air pump works by means of a piston that moves up and down (could also be side to side) inside a cylinder. It admits air into the compression chamber at the top of the stroke and forces it out the exhaust valve or port as the piston moves down. The larger the diameter of the piston, the more air is inside the compression chamber to be compressed. The longer the piston stroke, the more volume inside the compression chamber, as well. So those two things determine the amount of air that gets compressed — the diameter of the piston/compression chamber and the length of the piston stroke.
As the air is compressed, its pressure rises. This is felt as resistance on the pump handle. The higher the compression, the greater the resistance. Small diameter piston = lower compression/resistance. Larger diameter piston = greater compression/resistance. Therefore you can compress more air with a large-diameter pump piston, but the resistance will build up faster. And a smaller-diameter pump piston lets you compress air to higher pressure, though less air is compressed with each stroke.
The question is, to what pressure do you want to compress the air and the answer is another question — how much do you weigh? Because when you reach your weight, it isn’t easy to pump any more.
But wait — there’s more!
Hold on, B.B. You said that the length of the pump stroke also determines how much air gets compressed. Couldn’t the pump have a very small piston with a really long pump stroke, and therefore compress a lot of air to a high pressure? Why, yes it could! Mr. Conners would be proud of you.
So — how tall are you? You see, when the pump stroke gets longer than you can conveniently pump, all the fancy science stuff doesn’t matter. A hand pump with a 5-foot (1.5-meter) pump stroke isn’t very useful, is it? Not only is it hard to carry, it’s also difficult to operate when the pump handle comes up over your head on the up stroke! You need that handle to be low enough that you can lean on it on the down stroke. It really matters when the air pressure/resistance rises beyond a certain point. That point is determined by your height and weight.
Mechanical advantage is possible, and some vintage pumps use it, but they’re rare compared to the bulk of the pumps we know about. The single-stage manual pump I have described today is the most common design encountered in vintage airgun equipment.
Dennis saw where I wanted to go with this experiment, and he took up the challenge enthusiastically. He used two different vintage-type pumps of his own manufacture, plus the modern Axsor pump to check efficiency. He also made a 9 cubic-inch air reservoir with a built-in pressure gauge.
The test fixture Quackenbush made to ascertain pump efficiency. The pump that’s attached to the test reservoir in this photo is the smaller one with the 5/8″ piston head. The reservoir holds 9 cubic inches (about 147.5 cc) of air. Wit this setup the operator stands on the pump handle (left) and pumps the air reservoir up and down (right).
This is an antique hand pump. The reservoir — a ball in this case — is screwed onto the top of the pump between the handles (left). The operator stands on the pump base (right) and pumps up and down several hundred times to fill the ball.
That’s where I will leave it for today. In the next installment I’ll tell you the results of our testing, and also why the three-stage hand pump is so good.
Don’t forget that this work was done to discover the potential power of antique big bores. We are concentrating on the hand pumps, but that was just part of what we were exploring.