SpudFiles

Just tested the project mentioned here, works nicely. Here it is at 10x filmed at 420 frames per second and 14x filmed at real time:

[youtube][/youtube]

As you can see power isn't spectacular, a 1 gram 0.22" pellet won't go through the base of a soup tin - but considering the 45mm long barrel and 3.5mL chamber volume it's hardly surprising.

Still, the concept works. Also of note is that I used a larger version of my originalfill rig to fill it, meaning the schrader valve is superfluous and I can simplify the design to this.

Here's the internals excluding the PVC sleeving:



Here's the filling and firing setup:

A longer barrel will certainly help, what are your future plans for it?

Longer barrel obviously, but looks good.

Quick question JSR- I noticed in the diagrams of your fill rig it seems like you have to force the projectile back through the tubing that is meant to seal it- do you run into alot of issues with that since the tubing ID is slightly smaller than the projectile OD?

I'm glad to see the concept is sound. it's probably hard to gauge this, but is there any indication that the mix behind the piston is igniting, leading to a loss in power?

I might be trying out some designs myself now that my taser is finally here.

Gun Freak wrote:A longer barrel will certainly help, what are your future plans for it?

Nothing for this particular prototype, something happened after the 14x test and it's leaking through the barrel. The seal isn't exactly heavy duty so I wasn't expecting it to last anyway, this was just a proof-of-concept test.

JDP12 wrote:Quick question JSR- I noticed in the diagrams of your fill rig it seems like you have to force the projectile back through the tubing that is meant to seal it- do you run into alot of issues with that since the tubing ID is slightly smaller than the projectile OD?

The seal hold the projectile by friction, so it needs to be a tight fit - basically, if you can push the projectile in easily, then it's going to pop out easily. If you want to take it to high mixes, there needs to be a lot of friction, so it will be hard to push in.

Lockednloaded wrote:is there any indication that the mix behind the piston is igniting, leading to a loss in power?

The only way to test that would be to make a clear launcher (actually that would be an interesting low mix demo launcher along these lines) however the short barrel and small chamber would explain the lack of power.

JSR has got a spark generator from a gas cooker ?

I've been putting together a prototype with a 3/8" QEV, but I think I'll have problems with it opening too early because of the sealing face to piston seat is so large. It is good because the cup-piston works as a one way valve, so the mix behind the piston has a very low chance of igniting

POLAND_SPUD wrote:JSR has got a spark generator from a gas cooker?

I knew you'd comment yes, found a reasonably priced source, works great and much more compact than the CRT.

Lockednloaded wrote:It is good because the cup-piston works as a one way valve, so the mix behind the piston has a very low chance of igniting

My worry would be that the heat would adversely affect the piston material.

jackssmirkingrevenge wrote:

Lockednloaded wrote:It is good because the cup-piston works as a one way valve, so the mix behind the piston has a very low chance of igniting

My worry would be that the heat would adversely affect the piston material.

I guess testing will tell all. The pistons aren't too expensive to replace, but I don't think it'd be any more of an issue than it is in standard piston hybrids with standard rubber sealing faces

is there a reason you went with .22 caliber barrel? It looks like your chamber diameter is around .75"-1", that much choking will drastically reduce performance. Why not go with .5" barrel? Is it for chamber volume reasons?

JDP12 wrote:is there a reason you went with .22 caliber barrel? It looks like your chamber diameter is around .75"-1", that much choking will drastically reduce performance. Why not go with .5" barrel? Is it for chamber volume reasons?

As a coaxial increasing calibre would mean significantly less chamber volume for a given cartridge size, and I'm trying to keep things compact. Besides 0.22" is an ideal calibre because cheap projectiles are readily available.

I'm also looking at ways to incorporate a pop-off element to make the design more efficient:

Wouldn't a pop-off design require a stepped piston in order to actually "pop open" instead of a slow hiss that builds up? Also, good luck finding a strong enough spring for high mixes, yet weak enough to allow fueling. I like the simplicity of the design though

Wouldn't a pop-off design require a stepped piston in order to actually "pop open" instead of a slow hiss that builds up? Also, good luck finding a strong enough spring for high mixes, yet weak enough to allow fueling. I like the simplicity of the design though

Lockednloaded wrote:Wouldn't a pop-off design require a stepped piston in order to actually "pop open" instead of a slow hiss that builds up?

It is a stepped piston, just a different geometry.

Also, good luck finding a strong enough spring for high mixes, yet weak enough to allow fueling.

You're right there, the design wouldn't work because the spring allowing the chamber to be filled (through the barrel) is the same keeping pressure in, so as soon as pressure was removed from the fill rig the cartridge would depressurise as well.

What exactly is a stepped piston? I know it has to do with the pop-off effect but is it the shape? Does it have to do with being airtight?