Help with a repeating pneumatic?

[Cthulhu]

I got the pneumatic bug again, and with the money I saved up over the summer I want to build myself a nice (and feasible) pneumatic cannon.
I'm trying to keep it simple, but with a durable metallic construction over the usual PVC that I'm used to.

The basic idea is to have an HPA (not CO2) paintball tank attached to an adapter which will screw in to an inline air compressor regulator to reduce the pressure, this will then be attached to a ball valve, which will then have a male to male NPT adapter connected to a pressure chamber. The pressure chamber has a solenoid valve between it and the barrel.
The general idea is that the ball valve is opened then close to put some regulated pressure into the chamber before the solenoid is actuated to fire the projectile in the barrel (1.5" diameter 29 inches long).
It should be noted that most if not all the working components will be slid into a pipe for aesthetic and durability purposes.

The problems I'm facing are some pretty simple pneumatic issues. What volume should the chamber be? How much pressure should I be using? Will the parts I've chosen work? Is rifling worth it?
Thanks so much in advance!

Here are the links to the parts I plan to use:
https://www.homebrewing.org/Paintball-A ... gJVFPD_BwE
http://www.homedepot.com/p/Rain-Bird-1- ... /100063198
http://www.homedepot.com/p/Husky-1-4-in ... 519174-_-N
http://www.homedepot.com/p/Husky-1-4-in ... /100039901
http://www.homedepot.com/p/Husky-1-4-in ... /205226498

[Sear]

Is the barrel gonna be metallic PVC-based?

What is the expected weight of the ammo? Will it be hard-hitting or rather fragile?

The ball valve seems excessive in the setup. Just bear in mind to leave a very good access port to the regulator. And unless you plan to use highly polished and precise ammo, I wouldn't opted for barrel rifling in the first setup. You can made a new barrel later.

[Cthulhu]

Is the barrel gonna be metallic PVC-based?

What is the expected weight of the ammo? Will it be hard-hitting or rather fragile?

The ball valve seems excessive in the setup. Just bear in mind to leave a very good access port to the regulator. And unless you plan to use highly polished and precise ammo, I wouldn't opted for barrel rifling in the first setup. You can made a new barrel later.

I'm thinking about using metal conduit pipe.
Probably going to be shooting foam projectiles.

I mostly just don't know how big the pressure chamber should be.

[Sear]

It depends on the operating pressure. And how much pressure does the projectile takes before it starts to disintegrate during the launch. Let's say that the volume of the pressure chamber around 0,35 l (12 oz) taking 16 bar (~230 psi) should work very well for you.
Mine has 0,5 l (17 oz) and shoots 25 gram projectiles at 12 bar far enough (over 130 yards with rather flat trajectory). I've got the double barrel length compared to yours. Pressure chamber output opening (and valve) is 1/2" on my setup.

What is more tricky is to find a suitable prefabricated metal pressure chamber for your needs; assuming you plan to use metal for everything. There are some prefabricated air reservoirs like this one https://www.festo.com/cat/en-us_us/products_CRVZS_VZS
The one with 0.4 l volume looks fine, albeit with 2" diameter; I'm just not sure of how much negative impact will the 1/4" opening have.

[Cthulhu]

The regulator I'm using only has a maximum output of 140 psi, so I'm assuming I'm going to need a much larger air chamber?

Considering this low pressure anything from Black pipe to copper or even PVC could probably hold it.

[Sear]

That is the correct assumption. You may try any inline low pressure paintball regulator. All of them should go above 200 psi easily and take 850 psi directly from a paintball tank.

[Cthulhu]

It's too bad I can't work with higher pressures, the solenoid valve I have only holds up to 150 psi and the ball valve to fill the chamber only goes up to 125!
It doesn't help that the price drastically increases with online parts for higher pressures.

[jackssmirkingrevenge]

I'm trying to regulate pressure from an HPA tank (800 psi) to 140 psi using an air compressor "in-line regulator". Would screwing the tank directly into the regulator be ok, or would it be dangerous to go from such a high pressure to a low pressure.

What regulators are you looking at? You need something made for high pressure, a normal regulator made for a shop compressor just won't cut it. The Home Depot reg for example will likely blow up if you feed it with 850 psi.

Some of the best and most reasonably priced on the market can be found at Palmer's Pursuit.

Most of them are already threaded directly for a paintball bottle. Speaking of which, I would highly recommend a Ninja bottle. You can modify the regulator easily by removing shims in order to lower the output to 450 psi, which makes it more efficient.

Second question: I'm kind of pressed for space on my design for the air chamber, what calculations should I do to figure out how small it can be? I figured that if I can get the same amount of molecules of air that are in the barrel in the chamber it should work (I did some old gas law formulas), but I'm not sure if I'm doing it right.

The best thing is to use D_Hall's excellent Gas Gun Design Tool to model your launcher virtually and play with the variables until you get the performance that you desire. A 1.5 inch barrel at 150 psi is going to need a fair amount of volume for what I would consider "decent" power, but your needs might not be as as stringent. Do you want to put a golf ball thought 1/4" of plywood at 10 feet, or through 1" of plywood at 100 feet?

[Cthulhu]

Thanks so much Jack!
I downloaded GGDT yesterday and have used it for hours on end, it's a wonderful application! However, I'm still struggling to find the proper inputs for a modified sprinkler valve.

Those Palmer pursuit stuff are nice, but get pretty pricey. Considering I mostly just want this build to shoot mini goal footballs I figure that an electric bicycle pump may be my solution.

I could hook up the electric pump to a battery pack and use it to pump up the chamber to my desired PSI. It's definitely not as good as a pneumatic system (I've always hated charging batteries), but it might have to do.