SpudFiles

Here's my idea: use a 3/8" brakeline barrel, 1/2"-3/4" copper pipe and fittings to make a piston valve gun. The piston would be triggered by using something like a cap gun hammer to pop the schrader open for a short period of time. The piston would have a strong spring behind it, and that coupled with the air cushion effect would return the piston after each shot. The leakage rate around the piston would have to be carefully adjusted so that it would fire but not get stuck open.
A FE would be the reservoir. Shoot 3/8" slingshot BBs. Use 200-300 psi. With an appropriate magazine, it would be semi-auto. What think ye?

And any suggestions for constructing a piston (for a small diameter barrel) that can take a pounding?

If you dont have a lathe BTBs neoprenen disk how to will work. ill get the link.
EDIT:http://www.spudfiles.com/forums/viewtopic.php?t=6214&

Solo wrote:What think ye?

HA! im not the only one who says it! :tard:

as for the piston, if you access to a lathe machine it out of aluminum. and aluminum can be bought off mcmaster.

Be careful using a fire extinguisher as a chamber. They have funny threads that arn't normal NPT. I had to get mine welded on.

As far as adjusting air leakage around the piston, try building a adjustable valve system directly into the piston. Like a pitcock with a lock nut to keep it from turning on it's own. Thats my suggestion for the piston. As for multiable shots on 1 charge, i beleave use of a hammer valve out of a bb gun was discused, look up "Hammer Valve" in the rite forum.

instead of triggering with a cap gun and a shrader... which i doubt will work

you could rig a pneumatic setup which i can guarentee would work TONS better.

@Scope: why shouldn't it work, and what makes your set-up better?

@jrrdw: I thought about using a bb gun hammer valve for the primary valve, but I wanted more flow than that. I wouldn;t want to use it as a pilot b/c I think the schrader/cap-gun would be easier. And I think my current concept would be easier to build than a homemade hammer valve.

for clarification, my FE would be the primary reservior. It would not take the function of the air compressor in clide's semi or markfh11q's auto, where the compressor hooks up to the pilot volume. It would be just like the chamber in an ordinary o/u gun.

The problem is that the spring to reset the piston and conserve air has to be so strong that it barely allows the piston to move back.
You are better off with a weaker spring bouncing the piston back and getting a significant boost from air from the rear chamber.
If it's just the spring versus whatever pressure remains in the firing chamber it is a very unequal battle.

why not use a pop off set to a pressure higher than your guns shooting pressure to trigger your piston valve.
Or

Because it wouldn't pop I would guess...

So Hawkeye, you are advocating attatching the reservoir through the pilot volume and having a seperate chamber to fire from, and somehow the reservoir pressure doesn't get into the pilot volume until you want it to return the piston and refill the smaller chamber?

One of my main objectives for this gun is to get very high effeciency. I want a high-power gun but I'm going to have to pump it by hand. So my hope was to use the pressure straight from the reservoir into the barrel, rather than using the reservoir to refill a smaller chamber, because that represents a loss of power and efficiency. (Whenever you expand air, you loose energy, and filling the small chamber from the reservoir expands the air.)

Hawkeye, also remember that I don't intend to vent the pilot fully. Since the piston should be several times the area of the barrel, I won't need to make too much pressure drop to actuate the piston. Also, my idea was to have a strong spring, but not to have much force against the piston while it is seated. That way, you can see the piston as more bouncing off the spring than being held shut by it. And it doesn't have to move very far to get full flow: just ask Jack!

But, all of this makes me think of something: what if there were a simple way to trigger something that would equalize the pressure in the pilot volume with the reservoir to aid the return? Something to where the piston would set it off when it slid back far enough. Hmmm. Hey....

What if the piston had something like a schrader valve in the back of it, so that there was a little pin sticking out that would hit a stop placed somewhere in the pilot area? Then the pressure would equalize between the front and back of the piston.

A shrader on the back of a piston would get pounded rite out of workable condition with in a few shots i saspect. How about useing flap valve inbetween the firing and main chambers. When all is equal it is closed, when the cannon is fired and the piston returns to the sealing position, during the return, the flap will pull open and fill the firing chamber again.
Just a thought off the top of my head.

Yes I am advocating that because it seems to make the most sense. When you pump the whole gun up(firing and smaller chamber) it will be at the same psi and then start dropping with every shot anyway. The gun will not even totally dump the firing chamber if the piston can reset fast enough and repressurise the pilot volume.
If you make the piston very large then it will have the whole volume and pressure holding it open unnecessarily long and waste air. I don't see how a piston and spring is going to win the battle against a wall of high pressure air.
Trying to use the schrader valve as a one way valve in the piston leaves it the wrong way around doesn't it? It would need to be pointing in front of the piston as that is where the pressure is coming from.
I still like my plan a bit better as it has already proven to work on a lesser scale.
If you figure out how much air is actually required to propel a very small projectile you may as well separate that amount into a smaller chamber.

@Hawkeye: "it has already proven to work on a lesser scale" Just for clarification, are you refering to a commercial product or a gun on this forum?

"The gun will not even totally dump the firing chamber if the piston can reset fast enough and repressurise the pilot volume." Good point. But you are still loosing some energy and I want to avoid that.

"leaves it the wrong way around" gaahhh, let me try to draw this up in paint, I don't think I'm explaining myself very well.

@jrrdw: I realize that, I said "something like a schrader valve" b/c I just wanted to explain that it would have a rod that would open the valve. And you missed me with that flap business.

I put a chamber behind the pilot on a small gun I had and got multiple shots although I didn't have a loading system or a sufficient quantity of extra air.
Now that I have pump that goes up over 300psi I can pump up a larger back-up chamber. I guess I could pre- fill a few to high pressure and hook them on with QD.

I think the "Gas Sampling Cylinder" Paintball gun on here works this way as well.
I'm not sure I understand why you say "lose some energy." It's unavoidable either way. I do know that the average piston gun is using way more air than it actually needs to shoot the projectile.
jrrdw just means a one way valve, but it has to have some help staying closed during the firing cycle. I think the piston moving back and temporarily holding it shut is the best way.