Re: Custom Marker [TANKS]

[Steve Edwards <sedwards-at-awger.net>]

adt22-at-drexel.edu wrote:

>Could that be related to porting? or is all that stuff bs?
>  
>

Not really related, but it's not all BS either. IIRC, my ProAm with an 
Armson bbl (rifled + muzzle brake) used CO2 at about the same rate as a 
shorter spiral-drilled bbl. Basically, the longer the bbl, and the more 
blow-by / venting, the more CO2 (or air) you'll use for each shot.

> This maybe true for comercial markers, But with the homemade ones you 
> dump alot more air behind the ball where as the comercial ones only 
> use a tiny amount of air.


Air works the same in a homemade marker as it does in a commercial 
marker.  :)

CO2 is more dense than air (44 gm/mole vs 30 gm/mole) and has 
drastically different thermal and expansion characteristics (in a full 
CO2 bottle, much of the CO2 is liquid).

Markers designed for CO2 expect higher input pressures (250-500psi), use 
tiny needle valves, and have a short impulse. Commercial markers 
designed for HPA (high pressure air) also use high input pressures.

Homemade markers designed for compressed air use lower input pressures 
(80-120psi), large-bore valves, and a longer impulse. Because of the 
lower pressures involved, it takes longer to pass the pressure through 
the valve and a "larger charge" is required.

This relationship is analogous to the differences between modern 
smokeless gun powders and "old" black powders. Modern powders burn much 
faster, generate higher pressures, and take up less volume for the same 
"power" as compared to black powder (compare the difference in muzzle 
energy between a .44 magnum and a  .44-50 -- same bullet, but modern 
powder pushes it faster than 50gr of old black).

Most of the tanks built to date use a cut-down commercial CO2 marker 
with a small (6-12oz) bottle. This can last all day if you're not 
shooting a lot, and I've never seen anyone fire so fast that they had 
any problems with freezing-up.

Most "air cannons" use PVC as a resevoir for the firing charge. There 
are multiple kinds of PVC, not all of which are pressure rated 
(non-pressure-rate PVC can explode violently under pressure). The 
pressure rating should be stamped on the side; if there's no pressure 
rating stamped on the side then it's not pressure rated, and you 
shouldn't use it.

The higher the pressure of your resevoir, the smaller it can be. "How 
much" higher and smaller is a matter of debate and in any event is 
limited by your primary pressure source (probably a 12V air compressor). 
My baseline is a resevoir with twice the volume of my barrel, 
pressurized to 100psi by compressor (rated for 120psi).

Theoretically speaking, as long as the resevoir ratio is maintained, a 
35" air cannon will have similar performance to a 25" air cannon at the 
same pressure, but will just be less efficient.

    - Steve "under pressure" Edwards