HPA RPG-7

[ED-SKaR]

You're over thinking it! The above wasteland shotgun was literally made between breakfast and lunch plus 30 minutes of assembling the plumbing bits a couple days prior. Unless you're really after making a replica of something, the fun part with these is letting your imagination run wild.

You have knowlege, skills and tools that I do not have.

please explain HOW, don't just say 'it's easy'

[renegadecow]

Ok, lets go back to basics. The general principle of this is no different from a pre-compressed pneumatic airgun: pressurized air is quickly released to propel a projectile. You have three essential parts: the barrel, valve, and air tank. How those are arranged are up to you as there are even more complex coaxial versions where the tank surrounds the barrel, but I'll not get into that because it makes my head hurt.

 

PVC

They come in all forms and sizes but for the most part are cylindrical or conform to cylindrical shapes. They come in standard sizes be it metric or imperial so a 1" pipe should fit in a 1" end cap regardless of brand you get. For our purposes you want to get PVC meant to hold pressure which is more often than not the kind used for water lines. Make sure you operate within the pressure limits of the materials you're using or it could end up blowing up in your face. Pipe and fittings are connected either by threading together or permanently glued with solvent cement. You can make things modular if you make parts threaded, but will eat up space and you need to use sealants for them like PTFE tape, liquid gasket or o-rings.

 

 

Barrel (red, 1)

This is just a plain tube, in my case a 1/2" PVC pipe 20" long. You'll see much longer barrels on things like potato/marble guns but we don't want/need to get into those velocities.

 

Valve (yellow, 2)

In this case it's just a 1/2" ball valve and manually operated by twisting it perpendicular or horizontal to the flow. You can get more sophisticated valves like solenoid operated ones but require a power source.

 

Tank (green, 3-8)

The air tank is basically everything that holds pressure behind the valve. I folded mine over with a couple elbow fittings so I'll label each individual part so you have an idea just what they do.

-3&4: Identical 1/2" elbow fittings to make a "u" bend. If you're going for an in-line build like my RPG in the previous page, you can omit these entirely. Note that these are joined together by a short 1" section of 1/2" pipe not shown because it's inside. 2-3, 3-4, and 4-5 are all joined by this method.

-5: Size reducer from a 1" diameter pipe to 1/2". Installed just so I can put in a larger volume pipe to compensate for being shorter. Ideally, a 1:1 barrel to tank volume ratio is enough for this purpose but it doesn't hurt to give it more air than it needs in case you want to propel heavier projectiles, in this case more BBs. Plus having the tank fatter than the barrel looks better.

-6: 1" diameter pipe, 19" long: This is the bulk of the air tank but is otherwise a plain section of pipe.

-7: End cap. It closes off the 1" pipe but this has been modified (hole drilled through) to accept a bolt-on Schrader valve.

-8: Bolt-on Schrader valve. An inlet port to be charged with a bicycle pump. If you want to recharge from a HPA tank you''ll need the appropriate fittings for that instead and make sure the air is regulated within the operating pressure of the receiving system.

[ED-SKaR]

Awesome, thanks, that helps a lot.

 

 

Noob questions: go!

 

Are all of the parts you mention from the same class of water pipe (1/2") including the ball valve? That would certainly make things easier to buy.

Also if I were to use a solonoid, do they come with the same threads or obvious thread adaptors to make them fit?

 

I think I'd prefer to use a normal airsoft inner barrel, and a loading breach (probably made from more barrel) that will feed 20-30 BBs. Using a normal airsoft barrel gives me the option of adding a hop unit.

To compensate for the thinner barrel ID, I can reduce my co2 regulator down to fairly low psi, but I guess testing it will be required to work out what FPS I actually get.

 

 

As for tools, there doesn't seem to be much cutting (only to fit the schrader valve/gas rig nozzle) and not a lot of fixing as it's all threaded so just resin to fix it once it works.

 

 

 

 

My current layout plan, for an almost rpg7 thing, would be to use a T-shape piece that goes backwards to the tank, forwards to the barrel and then down through the grip and attaching the co2 line there.

 

 

Thanks again

[renegadecow]

Are all of the parts you mention from the same class of water pipe (1/2") including the ball valve? That would certainly make things easier to buy.

Also if I were to use a solonoid, do they come with the same threads or obvious thread adaptors to make them fit?

All parts 1 through 4 are 1/2" spec. 5,6, and 7 are 1".

 

I think I'd prefer to use a normal airsoft inner barrel, and a loading breach (probably made from more barrel) that will feed 20-30 BBs. Using a normal airsoft barrel gives me the option of adding a hop unit.

Just make sure you secure the inner barrel down really well because even 100psi from one of these things can have that flying hard enough to shaft somebody.

 

As for tools, there doesn't seem to be much cutting (only to fit the schrader valve/gas rig nozzle) and not a lot of fixing as it's all threaded so just resin to fix it once it works.

There are certain procedures you need to follow when working with PVC. While it's pretty much cut and glue you need to:

-make sure pipes to be joined are cut square so the adhesive spreads evenly

-leading edges of pipe should be slightly beveled so it doesn't just scrape the adhesive away when joining

-follow instructions on the solvent cement about how long to hold glued pieces together and cure times (personally I don't use solvent primers because the cement I use is nasty enough as it is)

-work fast but don't rush

[renegadecow]

Video of it in use.

http://youtu.be/___2_KjoQec

[Mike 8-{>]

I wonder.... would it be possible to do the tubes concentrically? Outer tube is outer wall of pressure vessel. Inner tube is inner wall of pressure vessel and barrel. At the muzzle the two are sealed together with an annular ring. Probably muzzle loader. At the breach an outer cap with the air inlet. The tricky bit is the actuator / valve. It would need to be mounted on the end of the inner tube and inside the envelope of the outer and cap. That said a solenoid valve could more easily have wires passing through the outer wall and maintain pressure integrity. Would allow you to have something looking like a bigger bore weapon with a more sensible bore for your projectile / payload and have the air reservoir built in?

[renegadecow]

That's what the coaxial builds are. Like a giant 40mm grenade in principle, but the tricky part with them is the shuttle valve or piston which is fabricated from scratch.

[paranoiddroid]

Ok, I have an idea for the rpg, that I need help with. (For a larp prop)

 

When fired what I want to do is simulate the back blast. To do this my idea is have a simple circuit attached to a 9v battery that when the thing fires (by twisting the valve) it puts a charge through the flash paper sheet/cotton held between 2 crocodile clips in the back bell.

 

Would something like that work?

[renegadecow]

It would, but since it's for LARP and you probably won't need a projectile, why not reverse the whole thing and have the back blast propel a handful of baby powder instead? The effect would be plenty dramatic whereas using something combustible will need plenty of the stuff (which can be dangerous) or will end up looking a bit lame.

[paranoiddroid]

I need the projectile, I need it to fire a foam rocket so I need the launcher guts still.

[paranoiddroid]

And I was thinking flashcotton/paper since I have the stuff around for conjuring tricks and it's plenty powerful and plenty cheap