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My smallish coax

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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My smallish coax

Unread postAuthor: Blitz » Wed Apr 09, 2008 7:52 pm

2" x 18" chamber
.75" x 16" barrel
2", 5.5oz piston made of steel and PVC (see description below)
Pilot valve: 3/4" QEV with 3/8" ball valve as its pilot

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So it's nothing remarkable, I was just happy I cut down on pilot volume by replacing my home-made 2" to 3/4" reducing bushing (the white one in the picture) with a one-piece bushing, shown below it. I also replaced the crappy blowgun with a 3/8" ball valve. Performance from these two upgrades made it quite peppy at 90psi. At 200PSI it just hurts my ears...

Believe it or not, the gun's two years old - - I just forgot to post it last year when I got it to work. Piston is made from two 2" steel washers, a 1.25" coupling placed between them, and two neoprene washers on each side (one as the sealing face and the other as a bumper). It weighs about 5.5oz, and making it lighter didn't seem to affect performance so I left it as-is.

You probably figured it out already, but the threaded street elbow on the QEV serves two purposes -- one, for directing the pilot exhaust valve air away from me if desired. And two, I put my hand over it when I fill up (since I fill from the chamber, not the pilot valve. I did this to minimize unecessary plumbing in the pilot). The threading in the front is for me to be able to remove the barrel and bushing it's housed in so I can either change hte barrel diameter or service the chamber. The threading in the rear is probably obvious to the rest of you -- piston service. I will be adding o-rings on the both of these threaded areas to improve air tightness (although there's no leakage right now).

And yes, I know I definitely need to clean it up. I had planned on painting it as well, and adding o-rings to the two threaded areas of the chamber. Believe it or not, the barrel is not glued in the bushing whatsoever, it's force fitted. I may end up reinforcing it, but even at 200PSI the piston hasn't pushed it out at all so far. I may replace the barrel with a sch80 version and toss in a paintball, just to see what happens.

I'll get damage pics/videos up sometime later this week or next.
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Unread postAuthor: Counterstriker » Wed Apr 09, 2008 8:06 pm

Nice gun! Is it a 2 fill gun? You fill air behind the piston than in the chamber itself?
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Unread postAuthor: Blitz » Wed Apr 09, 2008 8:08 pm

Nah, it just goes in the chamber directly. The QEV is why I put my hand on the exhaust until the chamber hits 20psi, then I can let go. Otherwise the diaphragm won't seal. :)
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Unread postAuthor: jrrdw » Wed Apr 09, 2008 8:13 pm

Did you have a press to use for pushing the barrel through the bushing? Will be glad to see the pics. Nice build.
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Unread postAuthor: STHORNE » Wed Apr 09, 2008 8:16 pm

Small? Sorry to tell you, but that coax isn't that small lol.

but still a very cool coax indeed. very uniform and neat, i love it.
price for parts total?

again, nice work. can't wait to see more
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Unread postAuthor: Blitz » Wed Apr 09, 2008 8:17 pm

I did at first, but then I used a rubber mallet to get it to my desired length. If I end up replacing it with a 3/4" sch80 barrel, I will have it pressed in all the way.
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Unread postAuthor: STHORNE » Wed Apr 09, 2008 8:24 pm

price for parts total?
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Unread postAuthor: Blitz » Wed Apr 09, 2008 8:26 pm

I guess for all retail, it'd be around $35. The only thing I actually bought was the 2" x 3/4" bushing, which was 4 bucks.
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Unread postAuthor: starman » Wed Apr 09, 2008 8:40 pm

I love coaxials...make me feel all warm inside... :roll: Well they do for me I guess because it's such efficient use of space and makes for a gun that's easy for the shooter to manage.

Nice little unit there. I'm glad you explained the elbow in the back, I wasn't getting it's purpose at first.

You shootin' paintballs, marbles, all of the above? At 200 psi, I'm sure damage is quite significant for small bore.
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Unread postAuthor: STHORNE » Wed Apr 09, 2008 8:44 pm

not too bad.

i would consider a camo paint job and maybe a custom wooden handle? It would add some individuality to it and look bad ass lol
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Unread postAuthor: Blitz » Wed Apr 09, 2008 8:46 pm

In all honesty I haven't shot anything of significance besides a lever arm which has a 3/4" diameter ball knob on the end. Everything else has been smaller with plastic bags used as wadding. I'm not a big fan of using wadding though, because I tend to push it beyond the barrel and into the chamber. lol Although, I could just put a small bolt through the rear of the barrel to prevent that.
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Unread postAuthor: STHORNE » Wed Apr 09, 2008 8:47 pm

jeez, pretty good performance from a "smallish" coax lol.

little package....big punch
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Unread postAuthor: Blitz » Wed Apr 09, 2008 8:59 pm

I try to keep the pressure below 120psi, only because I don't trust the fittings all that well. And even at 120PSI, GGDT predicts only 102 ft/lbs of muzzle energy with a 3' barrel and a 1oz projectile so it's really not that powerful. :)
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Unread postAuthor: Counterstriker » Wed Apr 09, 2008 9:10 pm

It looked like a double air fill cause the thing next to the gauge.. but that was before I read the description :D
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Unread postAuthor: Blitz » Wed Apr 09, 2008 9:12 pm

Oh, yeah that's the standard schrader valve I inserted to allow fillups from smaller devices, otherwise I prefer quick-connect couplers.
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