First pneumatic cannon- Well on it's way (update with video)

[niglch]

Well, my friend and I started planning this gun around the end of last December and have been steadily working on it whenever we could. I have to admit that we aren't very experienced as the only other potato cannons we built were simple spray & pray combustions. As fun as these simple ones were, we wanted something more.
In spite of our lack of experience, we decided to go for something cool: a piston-valved, bolt-action sniper rifle. Most of the "sniper rifle" designs I have seen use the somewhat less powerful sprinkler valves, so I wanted to see if I could make something rifle-like using a piston valve. I've looked around on this site and haven't seen anything designed quite like this, so hopefully it will at least be a little interesting and different. It was definitely a challenge to design, build, and finally get in working order, but it's finally paying off. So, after countless trips to multiple hardware/plumbing stores, many newbish posts (sorry guys), and many hours of labor, I finally feel this gun is worthy of being shown off a bit. It is still a work in progress, but now that it is functional and starting to look like what it will in what I envision as it's final form, I have decided to post it for some critique.
I feel the most unique feature is the way we mounted the stock. It's basically attached with a DVW endcap with the square head sawed off to make it flat with a hole in the center through which the pilot system was installed. This end cap was then epoxied to the pressure-rated hex-plug which makes up the back of the piston valve. The entire thing can basically be disasembled if something goes wrong. The bolt action breech loading design basically follows the how-to which is on these forums, but it is slightly adapted for a piston valve porting. By the time this gun is completely finished, I'm hoping to have a nice black paint job, a scope (mounted who knows how), and a rifled 1" barrel (I also have to figure this out).
I'm looking forward to your comments or suggestions!

[OuchProgramme]

Ahh good to hear that the WXY is not being pressurized
.

Or else..i would have to call in the DWV police!

[Gippeto]

You spent some time thinking this out, and it shows.

Good job!

How about some specs?

Piston pics and details, pilot size, chamber size, etc?

[bigbob12345]

Hmm
Lucky DWV thing isnt unde any pressure otherwise Id be all over your a$$.
Next time just use a pressure rated tee in place of that DWV thing, they look a lot better.

But nice job
A larger pilot valve would of been better but you can always do that next time
Again I must comment on its overall niceness

[elitesniper]

Wow you done your homework! very nice, might I recomend a larger pilot 1/2 or up will give you more ''ommph''

[Hydra]

That is one of the best guns ive seen in terms of aesthatics and comfort. Simple, strong (piston valve), and looks awesome.

[niglch]

Thanks guys. The only problem I'm noticing right now is that at higher pressures (70+ psi), there is a slight delay (1/2 - 1 sec) from when I flick open the pilot valve to when the gun actually fires. I'm guessing this means there's still a little too much air flowing past the piston. It's basically made out of a 1 1/4" endcap filled with epoxy with a thick rubber sealing face. I wrapped it in some artists tape so it fit snuggly but maybe it wasn't enough. I guess I could always try more/different tape or figure out how to cast a piston from epoxy.

[elitesniper]

you need a bigger pilot valve like a 1/2 or 3/4 ball valve.

[MrCrowley]

You don't need a bigger pilot valve, but it will increase performance a fair bit.

[starman]

Hmm
Lucky DWV thing isnt unde any pressure otherwise Id be all over your a$$.
Next time just use a pressure rated tee in place of that DWV thing, they look a lot better.

FYI, BB is of course one of the highly esteemed resident DWV police....rules with an iron fist....

[niglch]

I made a video of the gun being test fired at different pressures. I don't have a chronograph so I can't say the exact muzzle velocity, but GGDT predicted 525 ft/sec with the 20g projectile used. It was made from a piece of sidewalk chalk cut in half and wrapped in duct tape. I tried using the sound from the recorded video and audacity to estimate the average velocity of the projectile and came out with something around 330 ft/sec at 120psi. I guess that means the gun is under performing by a good amount. The reason I avoided using a 1/2" ball valve was because it would have required drilling a very large hole in the PVC pipe. I don't really want to redo everything so would decreasing pilot volume provide an alternative solution? Right now there is probably between 5 and 7 cu. in., but I could probably get rid of about half of that. Hopefully the video will reveal something about the performance of the valve.
[youtube][/youtube].

[Biopyro]

Sounds like you have exactly the same problem as me. My pilot volume is currently pretty enourmous, and as such there is a big lag in piloting the piston and actuation. advice topic

[SpudFarm]

that is one of the best looing newbie cannons i have ever seen! (good for seniors to!)

the pilot valve needs to be 3/4" atleast IMO

[starman]

Just watched your Youtube...impressive gun there. Looks like you've made some improvements and added some things on there since you originally posted.

I like the breech load system you have and the wooden brace is well done.

Has a sleek but tough look to it. Has a solid sound to it too. Nice work!

[niglch]

Sounds like you have exactly the same problem as me. My pilot volume is currently pretty enourmous, and as such there is a big lag in piloting the piston and actuation. advice topic

I'm very happy right now. I was able to solve the problem by doing a little optimization to my piston valve without having to actually get a larger ball valve. I was able to reduce the pilot volume to what I estimate is no more than 2.5 cu. in by cutting a piece of wood to fill some of the extra space behind the piston itself. Also, I re-wrapped our piston with tape to make a slightly tighter fit. Now the time taken to pilot at 120psi has been reduced from .24 sec to .04 sec. The average speed of the projectile over 50 ft has jumped from about 410 ft/s to around 495 ft/s, which is much closer to GGDT's calculated muzzle velocity of 527 ft/s. The gun has a noticeably louder sound and more powerful recoil. Now just to add a nice paint job and scope.