25 Mar 2008 22:17:27
loquak
Design of a pre-charged pneumatic airgun

Hi,
I'm a machinist by occupation and got this wild idea about making my own
pre-charged pneumatic airgun (no legal restrictions in my country). The
sites of Gary Barnes and Dennis Quackenbush inspired me a lot. While I
have experience with firearms and airguns, I've never designed or made
one myself. So I had to resort to the newsgroups, hoping someone could
shed some light on me regarding a couple of design aspects.

I guess the basic principle here is quite simple: a long barrel, an air
reservoir that can hold up to 3000 PSI of compressed air, and an on/off
valve that works as the trigger. Plus a pump or a scuba-tank.

What would be the best way to implement the trigger mechanism? Does it
suffice to have a ball valve and turn it open as fast as possible (using
a spring) to allow the air to flow from the reservoir into the bullet
chamber? What kind of valve would do this job best?

How about if I wanted multiple shots from the same air reservoir, but
less power with each shot? How is this generally done? Some sort of
feedback-piston system powered by air in the muzzle that turns off the
valve?

I'm no physician, so is there a rought formula to computing the
approximate FPS when barrel length, reservoir pressure and volume and
bullet weight are known?

Any other important things that should be taken into account, to achieve
even somewhat good accuracy and power? Well, the system has to be
completely airtight - I guess O-rings will do the job as seals. The
material of the steel parts will be stainless steel (AISI 361) so that
won't be the bottleneck.

All kind of hints are welcome!


27 Mar 2008 03:46:39
Chris Braid
Re: Design of a pre-charged pneumatic airgun

In article <y5dGj.315636$H57.53501@reader1.news.saunalahti.fi >,
loquak <plz.reply@to.ng > wrote:

> Hi,
> I'm a machinist by occupation and got this wild idea about making my own
> pre-charged pneumatic airgun (no legal restrictions in my country). The
> sites of Gary Barnes and Dennis Quackenbush inspired me a lot. While I
> have experience with firearms and airguns, I've never designed or made
> one myself. So I had to resort to the newsgroups, hoping someone could
> shed some light on me regarding a couple of design aspects.
>
> I guess the basic principle here is quite simple: a long barrel, an air
> reservoir that can hold up to 3000 PSI of compressed air, and an on/off
> valve that works as the trigger. Plus a pump or a scuba-tank.
>
> What would be the best way to implement the trigger mechanism? Does it
> suffice to have a ball valve and turn it open as fast as possible (using
> a spring) to allow the air to flow from the reservoir into the bullet
> chamber? What kind of valve would do this job best?
>
> How about if I wanted multiple shots from the same air reservoir, but
> less power with each shot? How is this generally done? Some sort of
> feedback-piston system powered by air in the muzzle that turns off the
> valve?
>
> I'm no physician, so is there a rought formula to computing the
> approximate FPS when barrel length, reservoir pressure and volume and
> bullet weight are known?
>
> Any other important things that should be taken into account, to achieve
> even somewhat good accuracy and power? Well, the system has to be
> completely airtight - I guess O-rings will do the job as seals. The
> material of the steel parts will be stainless steel (AISI 361) so that
> won't be the bottleneck.
>
> All kind of hints are welcome!

Hi

The valve is normally a poppet valve, like in a car engine, with a
diameter around 1/4 or 3/8 inch. It sits on a semi-soft seat, such as
Delrin. A hammer driven by a spring hits the valve stem hard enough to
overcome the air pressure for a few milliseconds, releasing a blast of
air into the chamber behind the pellet. To adjust the muzzle velocity,
change the strength of the spring, or amount of spring compression, or
the mass of the hammer, or some combination. The valve stem should be
hardened at the tip, so it doesn't get mashed out of shape. Quackenbush
has good info on what alloys are best used here and elsewhere.

You should put a regulator between the poppet valve and the reservoir,
to knock the pressure down from ~3000 psi to ~800 psi. A properly
designed regulator will provide constant pressure, thus consistent
velocity from shot to shot, until the reservoir pressure falls below the
regulator setting. Pressure regulators are pretty simple. A PCP's
regulator and SCUBA high pressure reg are essentially the same design.
If you aren't sure how they work, see if you can get the tech guy at a
local dive shop to show you the innards of one.

If you run the gun off CO2, you don't need a regulator. CO2 going from
liquid to gas provides automatic regulation.

It's important to minimize the free volume between the poppet valve and
the back end of the pellet. You also want the air passages to be as
large diameter as you can manage, for free airflow.

Pneumatic spudguns operate on much the same mechanisms as PCPs, albeit
at much lower pressures. There's some theory to be found at

http://spudtech.com/content.asp?id=6

The spreadsheet at the bottom of the page is a not-bad model of the
basic physics of a spudgun. It's less adequate as a model for PCPs
because it doesn't account for the effects of compressibility and choked
flow, but it should offer some insight if you don't expect too much of
it.

The hard part is going to be rifling the barrel. Guy Lautard sells plans
for a rifling machine.

http://www.lautard.com/

Have fun...

- Chris


26 Mar 2008 20:55:28
Long Ranger
Re: Design of a pre-charged pneumatic airgun

> The hard part is going to be rifling the barrel. Guy Lautard sells plans
> for a rifling machine.

There ought to be any number of inexpensive rifles to be had just for the
barrel, that can be adapted to your project. For instance, 10-22 barrels are
often given away by people who specialize in outfitting those guns with
custom barrels.