Alright, I started this thing last year and I hit some major problems, but now I really want to finish it, so I'm looking for as much help as I can get. Basically, I'm building a gun to be fully automatic. It runs on a piston-driven blowback system, like that of an AK-47. The explosion pushes a piston in the barrel out, which opens the breach, allowing another round into the barrel, opens the gas, and turns off the spark. Then, a spring pushes the piston back in, closes the breach, turns off the gas, and hits the spark, firing another round and starting the process over again. It will (probably) have a gravity fed, open ended magazine. I'm running the gun off of propane and oxygen, and will have a sealed and semi-pressurized chamber. The ignition will be from a spark plug out of a car, running on a 12 volt car battery. The gun is made from PVC, and has a 4 inch, 2 foot long chamber, and a 1 1/2 inch, 4 foot long barrel. I'm not firing potatoes because you can't get them in a uniform size without spending way too much time carving them. Instead, I'm taking ping-pong balls and using a needle and syringe to inject them with water, then freezing them. So I suppose it is less of a spud gun, and more of an ice ball gun.
If I lost anyone in that description:
http://www.putfile.com/pic.php?img=3744986
http://www.putfile.com/pic.php?img=3744985
That is a picture of the gun last year when I sort of gave up on it, as well as my original plans. In the picture, I only have the propane installed, and have none of the mechanical systems working. The pipe coming out the top is the "magazine." I fill it with balls, and they drop into the barrel, right in front of the chamber. It is hard to see in that picture, but there is a hole in the top of the barrel, and a slide that covers it. The pipe on the bottom is the piston. I currently have it cut to half that length, and used small steel bars to hook everything up.
So now that I have explained what I have done so far, I'll get to the problems and pray that one of you can help me. I suppose the most prevalent problem would be the fact that when I open the breach, one ball drops into place, but then there is absolutely nothing stopping it from rolling along the barrel and allowing another ball in. This would be fine if I wanted to fire 2 balls at once, however the second ball tends to get jammed as the slide closes over the breach. Any ideas on how to only let one ball in at a time would be great.
The second problem is that when I turn on the propane and oxygen, I don't think they will have time to mix and fill the entire chamber during automatic firing. I've heard about having a fan in the chamber, but I'm not sure how that works and if it would help me at all.
I'm sure I have more problems that that, but those are the two that are giving me the most trouble. Any feedback and/or ideas would be great.
Fully Automatic
For the dual loading problem, you could add some kind of device that will block the next ball in the magazine when the breech is open, and let them through when it is closed. That way you'll only have one ball free to drop into the barrel.
You could put a small computer fan inside the chamber and just keep it running constantly. Though a high rate of fire, a plastic fan may not stand up to the heat.
Do you know that the force you get out of the piston is enough to operate all your mechanisms? If it's not the fueling system could be operated electrically.
You could put a small computer fan inside the chamber and just keep it running constantly. Though a high rate of fire, a plastic fan may not stand up to the heat.
Do you know that the force you get out of the piston is enough to operate all your mechanisms? If it's not the fueling system could be operated electrically.
I'm having trouble designing the mechanism to only allow one ball at a time. Simply having a bar move into place and hold the rest of the balls in the loading tube doesn't work because it gets jammed too easily and will lock up the entire gun with the gas still on. I'm thinking my best bet would be to have 2 large disks, maybe a foot in diameter, mounted horizontally on top of the breach, with one on top of the other. The top one would have many short pieces of pvc pipe pointing strait up around the perimeter of the disk, and they would be just long enough to allow one ball to drop into them. There would be a hole in he top disk under every pipe, so if the bottom disk was not there, they would all fall through. The bottom disk would be solid, except for directly over the breech, there would be a hole to allow a ball to fall in. The top disk would have a type of ratchet system to allow it to spin one way every time the piston fired, but then not turn the other way when the piston went back.
I have no idea of that description makes sense to anyone but me, but describing it helps me picture how I'm going to build it.
As for the problem of the fan overheating, I live in northern Ontario, Canada and I can just pack the thing in snow.
I'm fairly sure the force of the piston will operate everything except I'm not sure about the propane. The valve is pretty stiff. How would an electric system work? You mean just rig up a small electric motor to turn the valve?
EDIT: Ok I found this. It looks like it would work for me, but when I checked the Product Data Sheet (link on the right side of the page) it said that the 'Cycle Time' was 6 seconds. Does that mean it takes 6 seconds to open, or is Cycle Time something else?
I have no idea of that description makes sense to anyone but me, but describing it helps me picture how I'm going to build it.
As for the problem of the fan overheating, I live in northern Ontario, Canada and I can just pack the thing in snow.
I'm fairly sure the force of the piston will operate everything except I'm not sure about the propane. The valve is pretty stiff. How would an electric system work? You mean just rig up a small electric motor to turn the valve?
EDIT: Ok I found this. It looks like it would work for me, but when I checked the Product Data Sheet (link on the right side of the page) it said that the 'Cycle Time' was 6 seconds. Does that mean it takes 6 seconds to open, or is Cycle Time something else?
Alright so I think I've gotten most of the major problems fixed. I made the loading device I described in my last post and it looks like it will work, but I don't have my camera right now, so I can't post pics yet. But now I've thought of two more things. First, since there will be (hopefully) less than a second for the gun to vent, I'm thinking of having a steady stream of air being pumped in. Not enough to blow the ball down the barrel, but enough to get a good propane/air mix. Any opinions on this? And second, how fast does a fan mix the air and propane together? And would two fans do it faster? I'm assuming they would, but has anyone ever tried this?
for your double feeding problem, perhaps a small flat spring in the barrel to keep the ball from moving up the barrel until fired, thus the ball above in mag remains in mag. . for an electric valve, perhaps a shut-off solenoid valve for automotive propane systems. require 12V power source. BTW, i find the concept fascinating, as my other hobby is building AK's.