6mm Coaxial Piston Cannon

[MrCrowley]

Just something I whipped up from spare parts in the garage over the weekend. Just a basic coaxial piston cannon with a 6mm barrel. I use it up to about 300PSI and everything seems to be holding together.

Specs:
Chamber: 25mm x 250mm long aluminium
Barrel: 6mm x 300mm long aluminium barrel from a weed sprayer
Piston: 15mm pressure rated PVC filled partially with epoxy and o-ringed using a hacksaw and dremel
Pilot Valve: homemade push button exhaust valve.

Performance is what you'd expect, probably shooting BB's around 250m/s according to GGDT. At 100psi (.12g) BB's were only going through one layer of cardboard with a blowgun as the pilot valve. So I got rid of that and built my own one.

It consists of a pop-off valve body with a small diameter rod that goes through the middle of it with a rubber washer glued to the end. Basically what happens is you fill the gun, the washer is pushed back against the inside of the pop-off valve where it can't go any further because it's too big. This seals the valve and then to fire you push the small rod with the metal washer on it, up in to the pop-off valve.

edit:
new pilot valve housed inside a male quick connect fitting. Smaller surface area on the push-button 'piston' and I also added a spring. This thing works great, comfortable, powerful and I can shoot one handed fairly steady.

edit 2:
Would I be right in saying I need ~7.3 pounds of force to open the push-button valve at 300 PSI assuming the 'piston' is 5mm in diameter?

edit 3: modded blowgun added and scope

[qwerty]

Looks great. How much does your homemade pilot differ to the blowgun?
because i think it would be a good project to build something like a push button valve.

[MrCrowley]

Well at 100PSI, the blowgun would manage to send a .12g BB through one layer of cardboard from a few meters. With the push button, the BB goes through 2 layers (only tried 2) and it bounced off and dented a piece of wood 2 meters behind it.

At 300PSI, the blowgun dents a 3/4" thick piece of wood with a BB, with the push button the BB get imbedded in the wood.

It took me about 10 minutes to make one in 1/2" fittings but it took me 3 hours to make one for 1/4" fittings haha. Mind you, with the 1/2" one, at 300PSI I had to bang the 'button' down on my work bench to trigger it

edit: when I use the track pump I guess I could trigger with the ball valve, but it throws of my aim quite a lot.

I also plan on adding an elbow to the pop-off push button pilot valve which might make it easier to trigger because i could push the palm of my hand in to it. But it's not so bad at the moment anyway.

edit:

I opended the box I was shooting at today to realise I was shooting through more cardboard than I thought.

With the blowgun, I was getting through 1 layer, stopping in a second layer. With the new pop off, i'm sending light BB's through 2 layers, denting the third, heavy BB's through 3 layers denting the 4th, and metal BB's through all 4 layers at only 115PSI.

[FORE!!!!]

very sexy,it looks a little like mine but without a blowgun,

whats the purpose of the bushing in front on the chamber attached to the barrel??

do you find you have to pull the push button rod initially to get it to seal?

[qwerty]

Im guesing it has a spring in it. Am i right?

[MrCrowley]

whats the purpose of the bushing in front on the chamber attached to the barrel??

No real purpose. I'm thinking of epoxying over it to give it a countoured look, like a bell reducer.

do you find you have to pull the push button rod initially to get it to seal?

No, I used to but I didn't like the idea of reseating the valve everytime I wanted to shoot so I kept fiddling around with rubber washers and o-rings until I finally found a combination that would reseat the valve by itself.

Im guesing it has a spring in it. Am i right?

Nope
Doesn't need one.

[MrCrowley]

Update:

New photos, new pilot valve. Improved slightly with a new housing, piston, spring etc.

Performance is slightly better than the other push-button valve I had on it. Probably due to the increased flow and smaller push-button piston.

[FORE!!!!]

Now thats more like it,very ergonomic

now you wont have as much trouble fireing at higher pressures, as long as ms palmer has been working out. "jokes" :

using the male connection from a quick connect is a very good idea,i think im going to steal it..

[MrCrowley]

as long as ms palmer has been working out. "jokes"

You mean Sherry Palmer from 24?

[c11man]

very nice pilot set up. i just curiouse how it took 3 hours to make the push button trigger. i made one the same size in abou 30minutes

[MrCrowley]

I had a hard time finding a 1/4" fitting that had a small 'seat' for the push-button 'piston', as the smallest 'seat' I could find in a 1/4" fitting would make the piston the same diameter as the inside diameter of most 1/4" fittings. Which means when triggered I'd have very poor flow from the pilot volume out the push button valve as the air would first have to get past the piston.

edit: you may have guessed by now that I have a very small amount of 1/4" fittings and they are hard to come by in NZ.

But somehow I overlooked these 1/4" quick-connect fittings I bought in America that never worked with QC's here, and they had a perfect sized seat allowing plenty of flow and they had a nice nozzle that the rod could fit down without limiting flow. Though I did take a grinder to the side (broke 6 drill bits trying to drill one hole in the side of it) to allow for better flow.

But yeah, the 3 hours contained numerous tests, which most worked but I had little problems like the push-button would have to be hand reset after each shot as if you just filled the chamber with air, it'd off-seat the push button and leak. And I experienced poor flow issues until I finally got this one working.

Reseats by itself and performance is good while mainting ergonomics and aesthetics, it's perfect

[FORE!!!!]

i know where your coming from after seeing your upgrade last night i made one in about 10 minutes ,but was having the same issue with flow. so i scraped the idea and just went back to my blow gun

[c11man]

oh i see, so if you have a hard time with 1/4inch fittings 1/8 is out of the question isnt it? cuz that s just what i used, a 1/4 by 1/8 bushing, i love america....

[FORE!!!!]

oh i see, so if you have a hard time with 1/4inch fittings 1/8 is out of the question isnt it? cuz that s just what i used, a 1/4 by 1/8 bushing, i love america....

Lol you should love america being the richest country in the world, you bloody yanks have eveything no offence
but the law of physics apply,for every country, what goes up must come down

[Technician1002]

edit 2:
Would I be right in saying I need ~7.3 pounds of force to open the push-button valve at 300 PSI assuming the 'piston' is 5mm in diameter?

Do the math. Unfortunately you have mixed metric an English units. The "piston" diameter is not the correct figure to use for the pushbutton. The force is calculated using the cross section area of the valve seat. This is a little smaller than the diameter of the piston and larger than the operating shaft under the button.

For sake of math the area needs to be calculated, but since forces in pressure and button pressure are in English units, it would be best to convert the dimensions to English and work from there.

1 millimeter = 0.0393700787 inches. 5mm = 0.196850393. First part done.

Area = Pi times Radius squared. Radius is 1/2 diameter..
R = 0.098425197 inches
Radius squared = 0.009687519
Pi times R squared = 3.14159265 X 0.009687519 = 0.03043424 square inches.

Force equals pressure times area.

Force = 300 X 0.03043424 sq in. = 9.130271904. So yes, your figures are in the ball park if your dimensions are correct. The force of any spring will be added to the result.


If I made a math error, feel free to point it out. I am human and do make mistakes once in a while.