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The Homemade Cobra

Because Shit Cannons Are Awesome

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#1 Buffdaddy

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Posted 30 June 2011 - 12:31 PM

The inception of this project was many weeks back, toward the start of the Homemades Contest. However, between work and simply not feeling well, it got partially assembled, then left in the corner. Last night, I went ahead and finished it.

Cobras are awesome for a number of reasons. You can single them and have fun shooting things hundreds of feet away (NOT AT WARS, THOUGH), you can easily shotgun them, and you can fire large ammo with ease. While not an every-round blaster, they are especially perfect for Defend the Core. These qualities I wished to maintain.

For reference, see THIS LINK for internals. As you can see, it's a backpressure system, and a highly efficient one at that. The trigger pull lets out a tiny bit of air from behind the piston, and then 750 mL of pressurized air shoots out the front. That ratio of pilot air to actual air coming out the barrel is astounding.

As it turns out, this system is easy enough to replicate!

Materials (looking for cheapest sources, naturally):
(9") 3" Sch. 40 PVC (~$.30) (Obviously, initial cost of 3" pipe is greater than this, since you get it in 5 or 10 foot lengths)
(2) 3" to 2" reducing PVC coupling ($6.24)
(2) 2" to 1/2" PVC bushing ($2.62)
1/2" Sch. 40 PVC
1/2" CPVC
1/2" PEX
(1) 3/4" PVC Elbow ($.34)
(1) 3/4" to 1/2" PVC reducing bushing ($.44)
(1) 1/2" to 3/8" Copper reducing coupling ($1.03)
1/4" NPT blowgun ($2-$6)
Bike Pump ($10 in my case)

Things you should already have, and last long enough to justify not including in the cost/incredibly cheap for the amounts used:
PVC cement
2-part epoxy
epoxy putty
E-tape
Craft Foam

Process:
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Let's start with the barrel. Simply hammer some 1/2" CPVC into the end of 1/2" PVC (roughly 11 inches of it), and stick a reducing copper coupler over it (1/2" to 3/8"). Easy enough. This helps keep everything straight, and gives a flat surface for the piston to seal against (alternatively, you could flatten the end of the plastic piping; my attempts at that failed, so I'v moved to manufactured fittings). Use epoxy to attach everything together.

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Here's what we use to get all that volume. 1/2" to 2" PVC bushings, and 2" to 3" couplings. We'll use a short amount of 3" PVC to link both sides (about 3" worth). Just a warning; once you reach 3" PVC, more often than not, you will find foam core DWV piping that is not pressure rated. It might hold the low pressures we deal with, it might not. But be safe and find the actual pressure-rated stuff. So keep that in mind. Also, ignore the really short barrel. It's an old pic. Sorry.

Assemble all that together with PVC cement, EXCEPT for the back of the tank. Now we get to the part that requires you to do some judging.

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Now for the piston and that bushing you have yet to attach: the small ridge of plastic on the inside of the bushing need sanded down, so that you can slide 1/2" PVC (with 1/2" CPVC hammered inside) all the way through. Put the cap on the end of the pipe, assemble without adhesive, and adjust until the piston only moves 1/8" to 1/4" (you can shove a pencil through the barrel, and measure its movement). 1/8" seems to achieve best results. If sealing is still a problem, then your fit is too loose. Wrap some e-tape around the PVC and lube it up.

Edited by Buffdaddy, 05 July 2011 - 12:12 AM.

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#2 Buffdaddy

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Posted 30 June 2011 - 12:57 PM

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For the back of my chamber, I used a 1/2" to 3/4" bushing and a 3/4" elbow. I then drilled a 1/2" hole for adding the trigger. For the trigger, I used 1/2" PEX and a blowgun from Harbor Freight (go cheap, yo). Use epoxy and PVC cement where appropriate. To increase the flow on your blowgun, follow THIS LINK (I only had to do the second photo).

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Attached to the real of the chamber.

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And the final picture. I have a Bell Airstrike Dual-Action pump shoved all the way into the 3/4" socket. PLumber's epoxy holds the trigger assembly steady.

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http://www.youtube.com/watch?v=B4pzhm4-oMQ
Ranges:
1" Stefans: 60-80 ft. With hopper and optimal barrel
6 1/2" stefans (shotgun blast): 40-60 ft, with roughly 5-8 foot spread

Edited by Buffdaddy, 10 July 2011 - 07:02 PM.

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#3 Buffdaddy

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Posted 30 June 2011 - 01:09 PM

POST RESERVED FOR EXTRA INFORMATION/RANGE TESTING/ETC

Volume: ~700mL.
-If done entirely from 2" PVC, this would have been roughly 350 mL, or just short of a titan tank.
-The 3/4" CPVC endcap will fit inside any PVC tubing 1 1/4" and above. For rough chamber volume calculations, use the following (assuming you use my construction method, etc, I've got it all worked out. Sch. 40 is assumed.):
1 1/4" PVC: 0.9415 in^2 times chamber length.
1 1/2" PVC: 1.4816 in^2 times chamber length.
2" PVC: 2.8014 in^2 times chamber length.

-If you go thinwall, then SDR 21 1" PVC will just barely clear the 3/4" CPVC coupling. Given the narrow fit, I would recommend putting the piston at the rear of the chamber, as in a situation like this, letting all the air out quickly is far more important than minimizing dead space behind your ammo. Larger sizes, you can proceed as instructed above.
1" SDR 21 PVC: .0519 in^2 times chamber length.
1 1/4" PVC: 1.1708 in^2 times length
1 1/2" PVC: 1.7156 in^2 times length
2" PVC: 3.0057 in^2 times length

As an example, here's a pistol made with this BP setup and 1" thinwall, and my homemade blast button:
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It has way too much pilot volume, but the homemade trigger dumps well enough that it still gets 60-70' ranges.

Also, for those of you who prefer other volume measurements, 1 in^3 = 16.387 mL

COST:
Right now, if I have everything calculated, then it's about $25, including the pump. If you add an OPV, then it gets more expensive. ($4 off Mcmaster, although you can make a homemade one, just use the search function at the top of the page)
Obviously, the cost can be reined in by not jumping higher than 2" PVC (16" long chamber would have the same volume, and cost several dollars less, by virtue of replacing the 3" reducing couplings with straight 2" couplings) and by finding cheaper pumps (I already have an idea for homemade pump in the works....)

SUGGESTED IMPROVEMENTS
-Wider orifice leading to the barrel, so you can better power your ammo. And also fire off balls.

Edited by Buffdaddy, 20 August 2011 - 11:16 PM.

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#4 arfink

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Posted 30 June 2011 - 01:50 PM

Looks to be a nice size for shooting big ammo.

BTW, that bell bike pump also fits snugly into 1" PVC with a little bit of sanding. I use that one all the time.
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#5 thesoxfan1234

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Posted 30 June 2011 - 01:57 PM

This is Grrrrreat! I love how you used the air compressor trigger as the trigger.
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#6 Amp

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Posted 30 June 2011 - 02:04 PM

Kinda looks like a Lighting Blitz, but anyways good job
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#7 karukiiz

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Posted 30 June 2011 - 04:54 PM

Do you expect ranges to be as good as a cobra, even with the 50mL decrease?
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#8 Buffdaddy

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Posted 30 June 2011 - 06:51 PM

I just took it out for some testing beside a Cobra, and it appears to be shooting just as well. However, the blowgun lets out the air slower than I'd hoped, which means there is a noticeable delay between trigger pull and firing. You can compensate for that, though. Nevertheless, I'm going to take a peek when I add the OPV, since that requires drilling a hole. I know there's more dead space I could eliminate to lessen the effect.
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#9 veginator

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Posted 01 July 2011 - 09:29 AM

This looks nice,but you may want to be aware that those two large bell reducers are DWV fittings. They are not rated for pressure what so ever and could blow on you after prolonged use. I would also like to point out your lack of using a pvc primer or cleaner before cementing the fittings together. I'm not trying too bash your gun, just trying to warn you so that you could avoid these in subsequent versions.
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#10 Buffdaddy

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Posted 01 July 2011 - 10:40 AM

This looks nice,but you may want to be aware that those two large bell reducers are DWV fittings. They are not rated for pressure what so ever and could blow on you after prolonged use. I would also like to point out your lack of using a pvc primer or cleaner before cementing the fittings together. I'm not trying too bash your gun, just trying to warn you so that you could avoid these in subsequent versions.


I am aware of that. As stated above, this could be entirely done out of 2" PVC or smaller. I just wanted to "compact" the design instead of having a 16" chamber to achieve the same volume. I've read up on DWV piping and fittings, and I'm aware that it's not pressure rated. That being said, it should hold for the relatively low pressures I'm dealing with. In any case, once I examine all the design flaws (I just pried apart the ends of the chamber to redo, then compare new results), I can make Version 2 and include all safety measures as would be needed.

- Using just cement (or in some cases, I've used 2-part epoxy before) allows me to pressurize the vessel, play around with it, and then at least have a chance to partially disassemble it for changes, assuming I have a vise and enough elbow grease. I'm not trying to encourage bad construction, by all means. This is just me testing things, then updating the thread later. Which reminds me, I need to update my previous contest entry this weekend.
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#11 veginator

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Posted 01 July 2011 - 03:47 PM

I am aware of that. As stated above, this could be entirely done out of 2" PVC or smaller. I just wanted to "compact" the design instead of having a 16" chamber to achieve the same volume. I've read up on DWV piping and fittings, and I'm aware that it's not pressure rated. That being said, it should hold for the relatively low pressures I'm dealing with. In any case, once I examine all the design flaws (I just pried apart the ends of the chamber to redo, then compare new results), I can make Version 2 and include all safety measures as would be needed.

I'm glad you know the dangers of using the the dwv. My spudding background just makes me cringe when ever I see those fittings on a pressure vessel.

Edited by veginator, 01 July 2011 - 03:47 PM.

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#12 Buffdaddy

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Posted 02 July 2011 - 09:29 PM

Quick update: I redid the chamber innards, so the piston is now in the rear, like a real Cobra. Also reduced/streamlined the pilot area, and even modded the blowgun a bit to increase efficiency. Once things are fully cured, we can test the update.

Also, I kinda view this entry as demonstrating my method of BP tank construction, not just a large blaster. As such, I will make sure to build a smaller version for this thread to show that it works, as well.
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#13 Nerf Gra

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Posted 04 July 2011 - 04:36 PM

the blowgun lets out the air slower than I'd hoped



Most air guns or blow guns are meant to blow air at high velocity so they have restrictions and shaped nozzles to facilitate that. Often times they are rated in cfm/m @ a certain psi. Perhaps if you look around you might be able to find one that lets the air go a little faster. I could be talking out of my ass but just trying to be helpful.

This thing is pretty awesome though. I am totes interested in a smaller version.
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QUOTE(VelveetaAvenger @ Dec 6 2010, 12:14 AM) View Post

Maybe there's no Mcmaster, but you could make the first coconut airtank.


#14 Buffdaddy

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Posted 04 July 2011 - 05:23 PM

Most air guns or blow guns are meant to blow air at high velocity so they have restrictions and shaped nozzles to facilitate that. Often times they are rated in cfm/m @ a certain psi. Perhaps if you look around you might be able to find one that lets the air go a little faster. I could be talking out of my ass but just trying to be helpful.

This thing is pretty awesome though. I am totes interested in a smaller version.


You're right; I compared this and another model, and it was the best. That being said, I did mod the blowgun for better flow. I'll include that in the next update.
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#15 Buffdaddy

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Posted 05 July 2011 - 12:15 AM

LEGIT DOUBLE POST

The writeup has been updated to reflect the new design! it now fires pretty much instantly, and seems much more insane.

More range testing to come!

EDIT: I also finaly got SDR 21 1" PVC. The 3/4" CPVC cap just slides inside! so I can finally test this method on it's smallest scale possible.

EDIT 2: Initial range testing says that my Cobra and the homemade are having identical performance. BOOSH!

Edited by Buffdaddy, 05 July 2011 - 03:41 PM.

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#16 Buffdaddy

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Posted 10 July 2011 - 07:02 PM

Video is now posted above!

Edited by Buffdaddy, 10 July 2011 - 07:02 PM.

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