userjjb

"5. Can NSF-DWV pipe be used for pressure?" -tyrant_bb
"5. No, only use pipe that's marked SCH40 or SCH80. Gravity feed fittings should not be used with pressure either." -CaptainSlug

You should be careful here making a distinction. Schedule-40 and Schedule-80 are actually only designations of dimension, they actually don't say anything specific about working-pressure of a pipe. In order to verify a pipe is rated for pressure applications look for "NSF-pw". Pressure rated fitings and pipe will always have this designation. HOWEVER, they also typically come with other designations as well. Virtually all pressure rated pipe and fittings will have a NSF-dwv rating as well (you could if you wanted use NSF-pw PVC for drainage, waste etc, but it's overkill). So just because the pipe is marked with dwv doesn't mean it isn't also pressure rated, just be sure to check first.

Unless you can set the breech to stay open for a set interval of time, I personally would recommend avoiding a brass slide breech for this kind of system because it has a rather high tendency to chew up darts. This is why I've switched to using a ramrod system in my most recent design.

And just so you know, that "3-way solenoid valve" or directional control valve is going to have a very low flow rate.

I could easily use a small QEV (1/8" or 1/4") in place of the solenoid valve if flow was a major concern, I just would prefer to be able to actuate the pilot valve electronically rather than pneumaticly so I have thin wires running to the trigger rather than tubing.

However, please note that the solenoid is not what's providing the power for the launching of the dart, but rather the barrel-sealing piston. The pilot (filling) side of the piston is pressurized and pushes the piston forward against the portion of 1/2" pipe sticking into the 3/4" tee. The neoprene on the front of the piston seals off both the pilot and chamber allowing them to pressurize. Once the exhaust port of the solenoid is opened the pilot area behind the piston vents to atmosphere and the chamber pressure pushing on the outer area of the piston slams it backward. This unseals the barrel and allows the chamber to vent down it propelling the dart.

The flow rate of the solenoid can actually be quite low and still properly actuate the piston valve. Especially with the o-ring and check valve you can pilot the valve with low flow. I have a 2" valve (that can be seen in my Spudtech thread referenced previously) that I can actually pilot with a Schrader (tire/bicycle) valve and that has around a 6 ci pilot volume behind its piston.

I agree however that the most problematic out of everything is getting the darts to feed smoothly, we'll just have to see how it goes once I get everything together. A great many people already uses breeches like this albeit not automated, so I don't anticipate it being to troublesome.

I may eliminate the internal bolt action and just extend the length of the 19/32" brass to reach the piston. I'll have one less part that can break/has to be made. This would also have the side benefit of allowing this piece to be removed from the inner section of 9/16" brass for disassembly/repair.

One idea I've been throwing around is having the opening of the breech actuated by the trigger pull rather than a spring. This again removes additional moving and external parts and increases simplicity of the design. I imagine the trigger-pull as a two stage actuation. The trigger would have some free movement backwards before it came into contact with the breech slide, during this time it would actuate the valve. Once the valve is actuated the breech would no longer be held forward and now that the trigger engages the breech slide it would continue to move backward, opening the breech and chambering a dart. Upon trigger release the pilot of the valve would resume refilling and the breech would close again. This would make the design less jam prone I think, but unfortunately the design would no longer be capable of full-auto. I may try the trigger breech return if the spring breech return doesn't work out.

BTW, CaptainSlug what do you think of the Gas Gun Design Tool (GGDT), have you tried it out yet? I would think it'd be right up your alley. It's a really quick and accurate way of predicting muzzle velocities before you go ahead and make the darn thing. It's accurate to with a couple percent, IIRC the only thing it doesn't take into effect is the transfer of heat from the working gas to the gun, the guy who made it is the Lead Firing Officer at the US's China Lake facility.

EDIT: Threw in some ideas for potential improvements and mades some comments about GGDT.

also the main problem is how to get barrel sealed from the air tank from the part of the piston, because they don't have something special for that.so that will take some time.

the operating pressure, don't know. i think something about 7-10 bar that is 100-145psi. but i don't know how many pressure my PVC can hold. so that will need to be tested first.

I know that some people have trouble finding proper PVC pipe or fittings in the UK, not sure about Holland though. My advice to you is to make sure that the piping and fittings you use are pressure rated. You want to use "solid core" PVC, it should have some sort of pressure rating on the side. I'd recommend trying to find a plumbing supply store, if anyone should have it, they would.

If for whatever reason you can't find pressure rated stuff, be VERY careful, try to pressurize it remotely to test it.

Advance wars?

Cool, what are you looking at for operating pressure? What are you planning on for chamber and barrel volumes? How do you plan to pressurize? Are you looking to achieve any particular range?