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Easy Backpressure Blaster & Foam Launcher Of Wonder

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#1 Buffdaddy


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Posted 01 July 2016 - 07:49 PM

On my recent trip to go warring in Canada, I got an idea in my head, and discussed it briefly at dinner with a few of the other Nerfers. Piston valve (or "backpressure") tanks rely on a small piston or poppet moving back and forth to let air into the tank, and then to let it out. The piston has to be able to slide freely. Kinda like those darts we put down our barrels at every war...

Let's get started, then!
Parts List
(1) 1 1/4" PVC tee

(2) 1 1/4" x 1/2" PVC slip bushings

(1) 1 1/4" x 1/2" PVC bushing, slip to threaded(f)

(1) 1/2" x 1/4" metal NPT threaded bushing (plus OPRV or gauge) (optional)

(1) 1/2" OD x .385" ID x 1" nylon spacer

(1) BoomCo dart (preferably new)

(1) Sch.80 1/2" PVC (only a few inches)

(1) 1/2" PVC adapter, male threaded to female slip

(1) 3/4" slip x 1/2" threaded PVC bushing

(1) 3/4" PVC check valve

(1) Hand air pump

(1) Shower diverter valve

(1) Thread tape

(1) 1 1/4" x 1/4" rubber washer

(1) 7/8" x 3/8" rubber washer

Solvent Weld and Superglue
Estimated cost: $25 - $35, depending on store sources used and setup. The bottom leg of the tee can just be a 1 1/4" plug, if you don't want to worry about having a gauge and you're using a pump setup that renders having an OPRV unnecessary. The solvent weld and thread tape can be used for other projects, but they're included in the cost here.

Build Guide

The Piston
After debating what darts to use and how to properly mount a rubber seal on them, I decided to go a different route. I superglued a brand new Boomco dart inside a 1/2" x .385" x 1" nylon spacer. In terms of mass and dimensions, this is actually almost functionally identical to the piston within an XBZ. And with most of the weight (what little there is) in the back, this will always travel straight and seal against the outlet.
Pump Assembly
There are several options available. Five Below has hand pumps at $5, with telescoping handles and a 1.05" OD, the exact same as 3/4" PVC. That combined with a PVC check valve can eliminate the need for an OPRV, with dead space before the valve setting the maximum achievable pressure. Some Bell Airstream dual action pumps can also go into PVC fittings, but being dual action, you're forced to add on an OPRV for safety since every stroke adds more air.

Trigger Valve

This is a shower diverter valve, found in the plumbing section of the hardware store. It can handle typical home water line pressures, and in our case is being used to vent the pilot volume. It has 1/2" IPS threaded fittings. The female fitting (the common line) will lead to our tank, while the pump and check valve will go on one of the legs. Depends if you want your blaster set up like a pistol or an XBZ.
Tank Assembly

In front of the trigger valve, add your threaded fitting, then carefully push the 7/8" diameter rubber washer. This acts as the rear seat for the piston, just to keep it from going back too far. Next, insert a stub of Sch.80 1/2" PVC. Add your 1 1/4" x 1/2" PVC bushing to that.


You can slide the piston into the Sch40 at this point. THE NYLON GOES FIRST.

For my firing tank, I used a 1 1/4" PVC tee, with a bushing assembly at the bottom to reduce the outlet to 1/4" threaded, so that a gauge or OPRV can be added.

The front PVC bushing had a 1 1/4" OD, 1/4" ID rubber washer glued in place. This is the surface the rubber on the dart head seals against.

And after that, you're done! Be sure to use solvent weld on the PVC, and thread tape on the trigger valve and various threaded fittings.


Finished Blaster


So far, it's been very easy to hit 90' consistently with a 4 dart hopper, and an operating pressure of 30psi.


To operate, make sure the diverter valve is in the correct position (the end of the switch sticking out indicates which leg is connected to the common). Pump up the blaster, then depress the valve switch to fire.


Improvements: spring-loading the diverter valve, and adding a real trigger.

Edited by Buffdaddy, 01 July 2016 - 10:56 PM.

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#2 CaliforniaPants



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Posted 01 July 2016 - 08:13 PM

what? nothing here is explained very well at all.

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trans as shit because fuck you

#3 The2ndBluesBro



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Posted 01 July 2016 - 08:16 PM

I found the only use for Boomco stuff! Seriously, though, this is pretty cool. Not sure if its supposed to be a write-up, but it's nice that homemade airguns are getting safer and more advanced.
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#4 Langley


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Posted 01 July 2016 - 10:02 PM

what? nothing here is explained very well at all.


Yeah, I don't disagree with that, but I think I get it.  It would help if the parts list were included, but it could definitely be explained better. 


So I think the thing on the bottom of the big Tee is just a pressure guage, that through me off a bit. 


The tee serves as an airtank.  When you pump the blaster, air flows through the trigger valve, and into this assembly, pushing the dart (piston) forward through the smaller rubber washer until the top of the dart seals against the larger rubber washer on the other side of the tee.  Then I assume air leaks around the back of the dart into the tee until the tank is pressurized.  When you pull the trigger, the trigger valve switches from 'pump -> tank' to 'tank -> vent'.  The dart is sucked backwards by the pressure differential, unsealing the tank and hopefully venting it's pressure into the barrel faster than it can leak back through the trigger valve. 


The biggest flaw I see with this design is that the smaller rubber washer is the only thing aligning the dart.  If the dart slides forward and doesn't seal with the larger washer, you're fucked.  Also the larger rubber washer should probably have a metal washer backing it, so that it doesn't deform as pressure builds and flex away from the dart. 

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#5 Buffdaddy


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Posted 01 July 2016 - 10:19 PM

what? nothing here is explained very well at all.




Yeah, I don't disagree with that...


Yeah, that's my fault. Somehow I uploaded the pics and not the rest of the text. Now here I am having to retype everything, with only minutes to the end of the night for the Homemade Contest.

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#6 CaliforniaPants



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Posted 02 July 2016 - 12:55 AM

okay this is much easier to understand now, I was most confused by that shower valve. I agree with langley that a metal washer to back the rubber is probably a good idea to keep a seal going, its a really interesting design though.

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#7 shmmee



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Posted 06 July 2016 - 09:16 AM

Great build! Back pressure tanks have always fascinated me and I've dissected every model of that style that I could get my hands on. I've also tried building my own but failed to produce anything functional. I did learn that the output hole to your barrel needs to remain a smaller diameter than the ID of the inlet hole. If the outlet hole is bigger, the piston wont move. I'm not sure if that's even possible with your design, but it might be a useful bit of knowledge for anyone trying to get too greedy with the output hole size.

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