Large bore (3 inch PVC) field gun

[Cthulhu]

I'm attempting to design a large bore field gun for paintball and I'm running into a few major design concerns.
I think that the use of a paintball CO2 tank (or two) released into a pressurization chamber before being fired with a sprinkler/solenoid valve would be best, but we'll the small size of the solenoid hinder performance for the enormous barrel (6 feet long 3 inch diameter)?

Also, what is the best method to make this breech loaded?

Thanks in advance, any help is useful!

[jackssmirkingrevenge]

I think that the use of a paintball CO2 tank (or two) released into a pressurization chamber before being fired with a sprinkler/solenoid valve would be best

You will also need a regulator to drop the pressure to 150 psi or less assuming you will be using PVC construction. Palmer's Pursuit make some nice stuff.

Will the small size of the solenoid hinder performance for the enormous barrel (6 feet long 3 inch diameter)?

You'll want a 2" valve and you'll want to modify it for manual actuation.

Also, what is the best method to make this breech loaded?

What do you intend to fire as ammunition? One possibility is to make self-contained cartridges but this can get a little complex. Then again, you could consider something like a burst disk:

Basically your cartridge is an empty tube with a burst disk made of a frangible material (eg aluminum foil) at one end. You load the cartridge, connect the nipple to a gas supply and open a valve briefly. The pressure inside the cartridge rises until the point that the burst disk fails, dumping the air through the barrel and taking the projectiles with it.

In terms of performance with a burst disk you will get maximum flow and virtually zero opening time, which means you can operate at lower pressures than you would have needed with a sprinkler valve, plus you have the realism of actually loading cartridges.

[Cthulhu]

So, I've been thinking about this for a while, and while I like your shell design, it has a few flaws that I wanted to work out. Particularly the need for an air compressor to fire the Cannon.

I came up with 3 experimental pneumatic shell designs (emphasis on experimental)

A- primitive design that uses a firing pin to pop the burst disk
B- uses a spring powered piston (blue gray) with an O-ring (green) triggered by a "primer" which uses a mechanism similar to a clicker pen (not displayed) to release the piston and compress the air to a critical point.
C- is a borderline against the rules solution, which uses a #209 primer (red) to not only burst the disk but also to agitate the air.

I'm not sure if any of these would actually work, but if any of them has a chance I'll test it and see if it's a step up in pneumatic breach loaders.

[Zeus]

I'm unsure of what you can construct and how large your cartridges can be, but look at the pressure differential triggered burst discs. If you can get it suitably compact it'd be the simplest solution and give quite optimal performance.

[Cthulhu]

I'm unsure of what you can construct and how large your cartridges can be.

They'll be 75mm diameter and constructed of welded steel (which will be pressure tested).

The only valve in them is a burst disk (depicted in grey), but the only way I can produce burst disks well is out of layered aluminum foil.

[Sear]

For the intended purpose, I would worry that the time required to pressurize the cartridge in order to burst the disc (few seconds) would be uncomfortable in real life scenario. Also you would need to connect the nipple and close the breech, which is one action more then ideal.

I would still give the solenoid valve a try. I have tested my HPA system (17.5 oz air chamber, 180-280psi usual operating pressure, very fast yet relatively small solenoid valve 1/2") on 60mm barrel (3 ft) few years back and I don't perceive any performance issues compared to my current barrel (37mm, 5.5 ft). I could high arc shots 200+ yds easily. Long barrelled 75mm gun is not too different diameter really. I could still feed it with my current setup I believe, rising the pressure above 280 psi. Even at lower pressure and with bigger air chamber then mine a very fast valve 3/4" or 1" should do the trick.

As for the simple breech loading mechanism, you may check "kugel brecher tank" setup on internet, on pbnation tank forum and on youtube. They often show operation of their main gun (2").

[jackssmirkingrevenge]

I would worry that the time required to pressurize the cartridge in order to burst the disc (few seconds) would be uncomfortable in real life scenario.

In practice, if the pressure source is disproportionate to the burst pressure (say feeding 850 psi from a CO2 bottle with a 75 psi burst disk) it would take less than a second.

If you want preloaded cartidges, another relatively easy method for high flow with a burst disk is to use a nichrome wire to pop a mylar burst disk as in this recently posted project. This gives you the benefit of electronic ignition meaning there is no complex mechanical trigger to make and your trigger switch can be anywhere.

[Cthulhu]

If you want preloaded cartidges, another relatively easy method for high flow with a burst disk is to use a nichrome wire to pop a mylar burst disk as in this recently posted project. This gives you the benefit of electronic ignition meaning there is no complex mechanical trigger to make and your trigger switch can be anywhere.

Wow! I can't believe I didn't think of this. but will there be a noticeable moment of time before the nichrome gets warm enough to melt the mylar enough?

This way I could help the general shell design but replace the inner mechanics with simple wiring inside the pressurization chamber.

[jackssmirkingrevenge]

will there be a noticeable moment of time before the nichrome gets warm enough to melt the mylar enough?

With a strong enough battery the right gauge of nichrome will glow red hot almost instantly.