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Analyze a gun's pump, & you'll know the gun's pressure & velocity
Article originally appeared in Airgun Revue #4
by Tom Gaylord
Thanks to Dennis
Quackenbush for providing the test data and the test pumps
used in this article.
With the current fascination for antique big bore airguns, hardly
a thought is given to the equally old hand pumps used to fill
them. Yet, without those pumps, the big bores could not have
existed.
In the scant airgun literature that exists, you can read a few
accounts of the performance of the old guns. In doing so, you
will encounter two very different viewpoints. The first includes
reports of people who have actually handled and fired vintage
big bores. They describe performance and give estimates of power
and accuracy that seem quite modest.
The other reports contain mostly from secondhand information.
These are much more effusive in their descriptions of the performance
of ancient air arms and have very little hard technical data.
Instead, they are full of subjective descriptions of what it
feels like (or must have felt like) to shoot the old guns - guns
that the authors may never have seen! That got me wondering.
Knifemaker and airgunsmith Gary Barnes is the one who started
me wondering. I saw the big bores he was making and listened
to him talk about their actual performance in relation to the
embellished tales he had read, until I, too, began to suspect
much of what has been written. The outside lock rifle he made
in 1998 seems to indicate a remarkable level of performance with
very little air pressure. Yet, when the wick is turned up in
the form of more stored air pressure, the performance goes down.
It seems there is a specific window of good performance for every
airgun, and on either side lies negative returns.
Higher airD pressure doesn't
always equal increased velocity in vintage guns
If the guns of the past were so narrowly regulated
by design, then their air pumps had only to get them into that
optimum range. Greater pressure did not equal greater power;
it actually upset a delicate balance of related components the
guns needed to perform at their best. So, when this relationship
is understood, the performance of the antique pumps can provide
clues about the performance of the guns they serviced.
I've seen vintage air pumps many times as parts of cased sets,
but I never paid much attention to their construction. The assumption
was, if anyone wants to charge an old gun today, that they'd
use CO2, which gets them up to as high as 900 to 1,000 psi effortlessly.
Or, they could fill from a scuba tank or a modern high-pressure
manual air pump like the Swedish Axsor. The antique hand pumps
are just for show, aren't they? Perhaps not.
If we can establish the performance parameters of vintage hand
pumps, then we'll also know the range of air pressures within
which vintage big bores operated. When that is known, their power
outputs can be calculated within an estimated range.
There are several good reports on vintage airgun power outputs.
But, woven in with them are the other reports that lack validity.
These are the exaggerated stories that many people, including
myself, have tended to believe and repeat because they are the
more spectacular accounts. With them go proclamations of accuracy
and of killing power at extreme distances, all based on these
dubious and unsubstantiated claims.
Before we proceed, let's look at another fantastic tale from
the world of firearms to illustrate what can happen when facts
are based on spectacular accounts. Legendary border patrolman
and writer Bill Jordan reportedly killed a criminal with a shotgun
at a range of 100 yards. He was shooting 000 buckshot (.36-caliber
shot), and one pellet was supposed to have hit the man in the
head, if the story is true. Lethal, yes! Probable? No! No one
doubts that the shot was anything but the wildest of flukes.
Even if the report is not correct, the fact that it COULD happen
is all that's important for this comparison.
If we accept an account like this, someone who read and believed
it might well write another article about how such a shot was
possible at even 125 yards. Then some other writer would key
on that and, before long, there would be claims of lethality
at 200 yards for a shotgun blast. It's pretty ridiculous when
examined this way, but that's what's happened with big bore airguns
for several decades, and no one has so much as raised an eyebrow.
Until now.
Vintage big bore guns did
their work at lower velocities than their modern counterparts
In 1998 came the science
of splatology. From Barnes' careful observations of recovered
lead balls fired from big bores, a correlation was made between
the size and shape of the recovered "splat" and the
velocity at which it impacted. With this information, it's possible
to determine impact velocity to within a narrow margin. Splats
from the past, recorded by cameras and drawings, can thus be
examined to reveal their impact velocity. It is, therefore, possible
to deduce how fast those old guns were actually shooting - which
turns out to be not as fast as some modern reports have indicated.
Still, splatology is not 100 percent conclusive proof. Balls
going faster than 700 f.p.s. disintegrate entirely, leaving no
record. There couldn't be any historic proof of those. And since
a splat represents only the impact velocity, we still have to
calculate how fast the projectile was going when it left the
muzzle. But a gun that shoots in the 550 f.p.s. realm is probably
not likely to also throw one over 1,000 f.p.s. Although we have
heard claims of very high velocities for modern big bores, we
have yet to actually see one demonstrated. In our experience,
it's been impossible, thus far, to reach such a speed with the
big bore guns made today, to say nothing of the less efficient
guns of centuries past. So, by knowing the air pressure limits
of vintage hand pumps, we have a big clue as to the performance
parameters of vintage big bores. (See note at the end of this
article)
One thing we know is that a pneumatic gun that functions well
at one pressure level will most likely not do well at another.
Therefore, if a big bore shoots 550 f.p.s. on a charge of 450
psi, it is very likely won't work at all when pressurized to
1,000 psi. We've known this about modern multi-pump pneumatics
for many years. If you over-pump them, their velocity decreases
until finally they cease to function at all. That's the point
called valve lock. Why wouldn't the older pneumatics, different
in caliber only, work the same way?
In fact, there is no other way they can work! All pneumatics
that use an impact-type valve will exhibit the same characteristics
of an optimum performance window within a certain range of pressure.
The window can be widened or even shifted by design, but there
will always be an upper boundary. If there weren't, the potential
energy of a pneumatic airgun would approach infinity, and we
know that it doesn't. By knowing what air pressure was available
to get behind the vintage big bore projectile, we can determine
the energy window. And we can find out the available air pressure
by studying the pumps that created it. That's the thinking that
went into this test.
Dennis Quackenbush suggested
the test
Dennis Quackenbush said his experience showed that actual tests
of vintage pumps showed things not suspected or discussed in
the literature. He felt the only way to know for certain how
well the viuntage guns and pumps worked was to build them and
test them exactly as they were meant to be used.
Empirical testing is
the only way to get the full story
Dennis Quackenbush has been making replica pumps for vintage
big bore guns for several years. His pumps are true to the old
designs, except that they use modern synthetic pump seals. The
old pumps had either a simple iron or steel piston that was lapped
into the bore of the pump tube or used stacked leather washers
on the end of the piston rod instead of a metal piston. The stacked
washers were compressed by means of a nut, so the fit could be
controlled.
I have encountered several dozen antique pumps, and all of them
were the type having the simple lapped iron piston. 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.
Before Dennis conducted the tests, it seemed to both of us that
neither vintage pump design could be the equal of one with a
modern synthetic seal; so, whatever pressure he could generate
with a modern replica would represent a maximum for any vintage
pump of the same physical specifications. That turned out to
be an incorrect assumption, as we shall see.
We both agreed that the practical maximum force that could be
applied would 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 or by jumping on the pump
handle, it isn't practical to do so - and it would be very hard
to do it on a continuing basis.
Mechanical advantage is possible, and there are some vintage
pumps that use it, but they are rare compared to the bulk of
the pumps we know about. The single-stage manual pump 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 manufacture, plus the modern Axsor
pump from Sweden to check efficiency.
How Dennis Quackenbush
tested the air pumps
Quackenbush connected the pumps to a 9 cu. in. test reservoir
attached to pressure gauge. He pumped up the reservoir and counted
the strokes required to get to certain pressure levels. He repeated
this experiment three times to verify his figures, which are
below:
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When you consider the number of pump strokes required to build
pressure in the 5/8" diameter piston pump, Dennis is not
going to repeat each test 30 times! But, in the repetitions he
did, the results were close enough to make the figures believable.
If anyone doubts the data, they can buy a pump from Dennis and
repeat the experiment themselves.
Next, he made another 5/8" diameter pump without a seal
to test the effectiveness of something truly vintage. I felt
sure he would not be able to reach the same pressure as a pump
with a synthetic seal. This one had only a tightly fitted piston
with a thin film of oil to seal it. The results were quite surprising!
The pump with no seal went all the way up to the same pressure
as the pump with the synthetic seal, proving, once again, that
we aren't as smart as we think.
Actually, it is incorrect for me to say this pump has no seal,
for the oil film seals quite well. Dennis achieved 720 psi with
the plain piston pump, using approximately the same number of
strokes as the other 5/8" diameter pump that had the seal.
Then, on a suggestion from one of his airgunning friends, he
tried "rapping" the air into the reservoir - similar
to the method used with the Korean-built Yewah Triple B Dynamite
shotgun. Rapping means imparting extra momentum to the pump through
inertia. With this method, Dennis raised the pressure ceiling
of the 5/8" plain piston pump to about 810 psi, but at a
high cost to his personal well-being. He said his wrists hurt
so much from doing the rapping just once that he had to recuperate
for several days thereafter. So rapping, while possible, is not
a practical way to fill vintage reservoirs to higher pressures,
and it's doubtful that anyone ever did it more than once.
One additional thing he did benefits this study even more than
these test results. He noticed that the piston diameter relates
directly to the total pressure achieved. By itself, this isn't
such a great discovery because Cardew already published similar
data in his book The Airgun from Trigger to Target. But
what Quackenbush did for us was remove the complex mathematical
formulae from Cardew's work and substitute simple equations in
their place.
Cardew says that 1,000 psi might be achieved by a heavy man.
Quackenbush's formula demonstrates that the weight of the person
doing the pumping is an essential component in the final pressure
achieved, as is the diameter of the pump piston.
Mathematical proof of
Quackenbush's tests
What the data show is that it is possible to calculate the maximum
pressure for any single-stage air pump by simply dividing the
weight of the pump operator by the area of the piston head. There
should be some loss of efficiency due to friction in the pump,
but this is most probably offset by some imprecision in determining
the person's body weight:
This
allows us to build a "theoretical pump" that can generate
whatever pressure we desire within the limits of physical laws.
For example:
Notice that the stroke of the piston does not enter into this
calculation. That's because the length of the stroke only determines
how much air is being compressed. It doesn't affect the highest
pressure that can be achieved. A longer stroke will compress
a greater volume of air; a small diameter piston will allow it
to build to higher pressure.
Up to this point, the term single-stage has been used when referring
to the pumps of old. Single-stage means that the pump
is compressing air in one direction rather than on both strokes.
In all cases, the direction for compression of a single-stage
pump is the down stroke. The upward stroke sucks in more air
for the next downward compression stroke.
The high-pressure modern air pump from Sweden, in contrast, compresses
air in both directions. It's really two pumps nestled one inside
the other, both housed inside an outer tube. Both pistons have
a small piston diameter. The center of the larger pump is occupied
by the smaller pump, so the AREA of the two piston heads is comparably
small. The larger of the two is the second stage, which operates
on the downward stroke - the direction in which the greatest
force can be applied. The upward stroke sucks the air into the
pump and pressurizes it slightly to begin with, making it ready
for the second stage to compress it to a very high pressure level.
This pump pressurizes a greater volume of air to a higher pressure
than its small piston would normally allow because it acts as
though it's twice as long as it is.
If a single-stage pump were made with a very small (perhaps 3/8")
piston, what sort of pressure might be possible from a 150-lb.
operator?
The small piston pump would take a lot longer to fill a reservoir,
so we might increase its stroke to help matters. That gives us
a long, thin pump that eventually becomes too long to carry conveniently.
Also, the thinner the pump piston, the thinner the piston rod;
although the piston HEAD might be able to compress very high
pressures, the rod that connects it to a source of force will
eventually become too thin to bear up under the strain.
From this discussion, we see that Cardew was correct. It is possible
to generate 1,000 psi and even more with a simple single-stage
air pump.
Not only do vintage pumps begin to max out at pressures much
lower than modern pumps can achieve, but the guns being pressurized
cannot deal with air pressures anywhere near these high levels.
Old guns use leather &
horn - not steel & synthetics - & function only on low
air pressures!
The large valve contact surfaces of vintage big
bores constrain them to use lower-pressure air. State-of-the-art
technology available to 17th century airgun makers allowed them
to seal valves against pressures ranging from 400 psi to 650
psi. They used horn and leather to accomplish as much as they
did. Today's use of hard synthetic valve heads bearing on thin
areas of contact on precisely machined steel seats was simply
beyond anything they could achieve.
They also lacked the homogenous materials from which to make
reservoirs to contain the higher air pressures. Even if they
could make the valves and pumps work at higher pressures, the
reservoirs would have held them back.
Again, I turn to the documented studies to support this statement
to find there's even less information than before. The number
of ancient airguns that have been intentionally tested to destruction
is almost zero. A fair number of them have blown up from too
much pressure, but that was accidental. Apart from a big boom
and possible injury to the operator, there isn't much to go on.
That said, Cardew does mention one of the folded and brazed
butt reservoirs that was tested to destruction.
Amazingly, the antique folded-iron reservoir held until 6,000
psi was developed. And, when it finally did blow, only one rivet
popped, resulting in a controlled exhaust rather than a catastrophic
explosion.
When such tests are conducted, the vessel being tested is never
filled with air for fear of explosion. Instead, oil or water
is forced in under pressure, so the failure can only result in
a safe leak. Unfortunately, this test is exactly the kind of
report that a careless person will cite when trying to operate
vintage equipment in an unsafe manner.
A case in point. Modern aluminum paintball tanks are designed
to contain CO2 safely. CO2 has a pressure determined by temperature.
At 70 degrees F, it is just under 900 psi. When the temperature
rises to 95 degrees, the pressure increases to around 1,100 psi.
Because of this variability and because of the possibility of
even higher temperatures, such as when a tank is stored in a
car on a warm day (130 degrees is easily possible), the manufacturers
rate the tank up to 1,800 psi. That's the number they put on
the label on the tank for everyone to see.
Along comes Joe Airgunner and reads this number. He thinks to
himself, if this tank is rated to 1,800 psi, it's safe to pressurize
it to that level. So he fills it with air instead of CO2. The
air tanks rated this high cost much more than the CO2 tanks (wonder
why?), so using a paintball tank represents a real savings. If
he does this, the tank is filled right up to its design maximum
- a level that was engineered by the manufacturer to handle an
emergency! Because CO2 pressure varies with temperature,
they make the tank to withstand a higher pressure in case the
temperature ever rises unexpectedly. It isn't expected to withstand
that pressure all the time, even though it is engineered to do
so. It is expected to hold a pressure of around 900 psi when
confined. But some folks just go by the numbers without appreciating
that they're actually subjecting these bottles to 100 percent
overfill. It doesn't end there.
Over-charge your guns at
your own risk!
If one person is willing to do that, what's to
stop another from pressurizing the same tank right on up to 3,000
psi with the same fill equipment? Maybe he's been using paintball
tanks this way for a long time, and there's never been any trouble
before. "After all," he says, "these things are
over-engineered, anyway."
Yes, they are. At 3,000 psi, you're more than two-thirds of the
way into that safety margin. Standard pressure vessels are engineered
to not fail with less than four times their standard working
pressure - which you'll remember is 900 psi, nominally. Blowup
can occur any time after 3,600 psi for a tank having a 900 psi
standard working pressure. That's for a new tank. Who's
to say when an old, abused tank will let go?
Tanks have safety valves (burst disks), don't they? Yes, but
do you want to trust your life to a small piece of metal that's
been sitting in the tank for years? What if the kid at the plant
decided to use heavier sheet metal when your tank was made? What
if there was a goof when the company placed the order and a stronger
material was used without anyone's knowledge? What if someone
in the field "fixed" it before you got it (perhaps
to keep it from rupturing so easily)?
If modern paintball tanks represent a danger from over pressurization,
what about vintage air reservoirs? They don't have burst disks
or any other safety devices, plus they've been lying around for
decades - and centuries - with their dubious metal walls containing
who knows what kinds of pressure.
The old reservoirs had grease intentionally smeared on the walls
near the valve to catch and retain dirt and dust. At 500 psi,
nothing is going to happen; but when you get one of these oldies
up to 2,000 psi or more, that grease is going to vaporize into
a highly explosive gas. Just ask a dive shop what happens when
petroleum-based lubricants are used in scuba tanks. Boom!
When we work with precharged airguns of any type, we want to
obey all the safety rules that pertain to them. When the airguns
are also large bore, we want to be especially careful because
they're so powerful.
How pressure relates
to velocity
The bottom line of this research is to provide some insight into
how powerful the antique big bore airguns are by knowing what
kind of air pressure they work with. There is no formula to calculate
such a relationship, and it may be such a complex relationship
that there never can be; but there is a fair amount of information
gathered from observation.
For starters, let's look at Quackenbush's Brigand. On CO2, we
get velocities around 575 f.p.s. with a .375-caliber lead ball
weighing 83 grains. CO2 is nominally 900 psi. With the same rifle
running on air at 1,200 psi, the velocity increases to somewhere
between 730 f.p.s. and 775 f.p.s. Pressurize the gun to 1,500
psi, and the velocity drops to around 600 f.p.s., where the high
pressure is lowered to the optimum range. What we can say about
the Brigand is that it has a valve that functions best between
1,000 psi and 1,200 psi - and it will function with limited results
between 600 psi and 1,700 psi. That's a broad range of pressure
but a much narrower band of optimum performance.
Looking at the outside lock built by Gary Barnes, we see an optimum
range of performance from about 500 psi to 650 psi, with a working
range of 300 psi to 800 psi. Although the outside lock functions
at about half the pressure of the Brigand, it gets a few more
shots on each charge of air. What's involved is a combination
of caliber, barrel length and weight of the projectile.
To achieve high efficiency with low air pressure, the valve needs
to remain open longer to allow air to continue to push the projectile
until it is free of the muzzle. A large caliber provides more
volume to lower the air pressure after it leaves the reservoir.
The farther down the barrel the projectile gets, the more volume
there is behind it; and big bores increase in volume faster than
small bores.
To push a heavy projectile fast, you have to maintain the force
pushing on it for as long as possible. That means a longer bore.
But a longer bore will diminish the air pressure behind the projectile
very rapidly unless the force is applied continuously.
What this all means is that big bore airguns must leave their
valves open much longer than small bore guns; and to do that,
they have to run on lower pressure. Note that when Quackenbush
went from 1,200 psi to 3,000 psi, his velocity increased by only
118 f.p.s. (775 vs. 893). To get that increase, he had to redesign
the valve because the standard valve wouldn't have functioned
at the higher pressure.
What can we learn from this? When we see a vintage .36-caliber
air rifle shooting a round ball at 675 f.p.s., we now know it's
about where it should be. Perhaps it might get up to 750 f.p.s.
A claim of 1,000 f.p.s. for a .65-caliber rifle with a 48"
barrel should be met with some skepticism because of what it
would take to actually achieve such performance.
The way antique big bore airguns are designed, there is no reason
to over-pressurize them. They only work well within the narrow
band of pressure for which they were designed and (sometimes)
tuned. By knowing the specifications of the pump used to charge
them, we know their operating range, and that reveals their performance.
__________________________________________
Note: In the fall of 1998, I fired a special Quackenbush Brigand
rifle in .375 caliber designed to operate on 3,000 psi instead
of the normal air/CO2 combination valve normally in the gun.
The combination valve operates best at pressures around 1,200
to 1,400 psi because it's set up to work with either air or CO2
gas; the 3,000 psi valve is optimized to that pressure alone.
The 3,000 psi gun shot an 83-grain round lead ball through the
chronograph at 893 f.p.s., making it the fastest big bore projectile
yet tested by The Airgun Letter. This gun got four usable
shots on a full air charge; but the first was the fastest, and
each successive shot was slower.