Paintball Gun

Yes, that's all: a 5" piece of 1" PVC pipe.

This is my design for a blow-forward, open-chamber, semi-automatic paintball gun with one moving part. Unless you count the spring or valve. It will be electronically controlled by a small circuit consisting of about five components.

It's not done yet. I still need to drill the air ports drilled from the valve through the body, and also drill the breech where the paintballs go in. The later will lie somewhere along the right-most red line - I'll decide exactly where once I decide which side is the top.

How it should work

Animated cutaway drawing

100 psi CO2 enters the rear port (upper left) and pushes the block (yellow) forward. The block pushes a paintball (purple) into the end of the barrel. When the block has moved forward 3/4" and has fully pushed the paintball into the end of the barrel and sealing off the breech, the plug (green) exposes the hole through the block, allowing the CO2 to pass through the block and drive the paintball out down the barrel. By then CO2 valve has closed, and the spring (red) pushes the block back to where it started, opening the breech and allowing the next paintball to fall into the breech, ready to be fired.



I designed this paintball gun to be built almost entirely out of four sizes of PVC pipe - 1" Schedule 40, 3/4" Schedule 80, 1/2" Schedule 80, and 1/4" Schedule 40. The 3/4", 1/2", and 1/4" pipe would have to be turned down slightly on a lathe so that all four pieces will telescope, which is why I required Schedule 80 pipe - Schedule 40 pipe has too large an inner diameter. Unfortunately, I could only find the 1" Schedule 40, so I turned the inner pieces out of UHMW polyethylene.

(For dimensions of PVC pipe, see Notes on Pipe.)

Front view - will be bored and threaded for a standard barrel

It still needs a barrel, though the opening is 0.680, proper size for a paintball, so it can be at least fired without a barrel, though I don't know how accurate or powerful it will be.

Rear view

The valve is made by SMT, part number VJ-3130, apparently discontinued. They make a comparable valve, but I forget the part number. I got this valve off eBay for $3 (actually, six for $18) including shipping, complete with gasket and wiring harness. Not bad. It is attached to the gun by a pair of tiny little screws, and sits on a ½" wide by 2" long flat filed in the PVC. I haven't quite figured out how to run the air channels through the PVC yet.

The valve will be controlled by a small 555 timer circuit set up as a one-shot timer (schematics for which are all over the 'net.) This should let the period that the valve is open be easily and precisely controlled, as well as making the gun easily adaptable to remote control.

Coat hanger spring on right, solder spring on left

I haven't been able to find a proper spring anywhere. I need one that is less than 1.033" OD, more than 0.724" ID, about 2" long unloaded, less than 7/8" solid length, and "of appropriate stiffness." For the cutaway, I wound a mockup spring out of solder (talk about a useless metal to use for a spring!). I tried winding a spring out of a coat hangar, but it's too big and WAY too stiff. I think I need music wire that is about 0.030" in diameter.

Update 2/28/2004 - Spring

Well, I made a spring. It's a bit stiff, but it ought to work. No picture, 'cause it looks just like the mockup spring.

Update 3/13/2004 - Barrel

I've installed a barrel and bored out the hole in the side of the body for the paintballs.

Ready to Fire - or it would be
if this weren't just a mockup

I bored out the whole for the paintballs too large. It's big enough, in fact, that if the first paintball rolls forward into the end of the barrel, there is space for a second paintball to wedge itself in behind the first, where it will either be crushed against the side of the opening when the block moves forward, or if it falls all the way down, follow the first paintball out of the barrel.

Block all the way forward, paintball
in end of barrel, block "unplugged"
allowing air through to fire paintball

The spring takes a force of about three pounds to compress to the full forward position. The area of the flange at the back of the block is 0.4263 square inches, so a pressure greater than 7 psi ought to be sufficient. I plan on using 100 psi, which should move the block with some authority!

Paintball in barrel

The barrel is an 8" long piece of 3/4" O.D. tube with a wall thickness of 0.035", leaving an I.D. of 0.680", perfect for a paintball. The barrel is Titanium. Why titanium? That is a very, very good question, one that really reveals your penetrating and deep understanding of the subject. Don't ever ask it again.

Block "unplugged"

The area available for compressed air to flow through the block is 0.21 square inches, the same as a ¼" diameter hole, though of course the greater edge length of the hole will make it equivalent to a slightly smaller hole (the edges create turbulance, which slows down the flow). At 100 psi, I'm hoping it doesn't matter.

Update 7/19/2003 - Success! Sort of.

I bought a bigger valve on eBay, since I began to have second thoughts about the both the size of the SMC valve, and how it would be mounted. This new one turned out to be a normally open valve (gotta read those descriptions better!), which proved problematic, but the real problem was that it didn't flow much air at the 80 psi my crappy little compressor would produce before blowing itself apart. But that's another story...

Gun with Asco valve

Although the gun did work, it did not work very well. I made a new spring out of lighter wire so the breech (blue part in the above cutaway) would cycle properly. Even so, it didn't have any velocity to speak of - firing at the wall only six feet away, most of the paintballs didn't even break.

Gun with Humphery's 310 valve

So, I replaced the valve yet again. This time I bought a Humphery's 310, which I haven't yet tested. I also plan on working on my compressor, to see if I get it to hit 100 psi before exploding.

As an aside, I found a chart of paintball sizes. My barrel is too small! Or is it? I miked a couple of paintballs and got some strange answers. One, that had sat out in the humidity was 0.70 inches. Others, still in the can, were 0.675 give or take.



© 2003 W. E. Johns