SpudFiles

I am building a .177 caliber bb rifle with a 0.5" piston valve and a 1.5' barrel (all metal by the way). My question is this: I was planning on making the chamber out of an 8" peice of 0.5" tubing, but i don't know if that is a big enough chamber to get the bb going at least as fast as a good pellet rifle. Also, my air compressor is a 5 gallon shop compressor that maxes out at 125psi.

i don't know if that is a big enough chamber to get the bb going at least as fast as a good pellet rifle

Have you considered getting a relatively cheap shock pump and using 300-400 psi? With smaller calibres especially pressure is much more important for power than chamber volume.

Just to give you an idea, I had modelled a virtual launcher on GGDT, using the same quantity of air but with ever decreasing chamber size (same diameter but shorter length) and therefore increasing pressure, and here's what I got:

20 inch chamber at 50 psi - 406 feet per second

10 inch chamber at 100 psi - 537 feet per second

5 inch chamber at 200 psi - 684 feet per second

2.5 inch chamber at 400 psi - 830 feet per second

1.25 inch chamber at 800 psi - 958 feet per second

0.625 inch chamber at 1600 psi - 1006 feet per second

This is a stupid question but what is a shock pump?

Something like this, keep your chamber volume to about 2 cubic inches and you can reach 400 psi easily.

I should receive mine today!! can't wait!!
Thanks Jack for the numbers, I didn't think such a small chamber could manage that speed...(even if it's a GGDT prediction...)

Good luck with it

I've begun to look into this idea again, now that I can use my paintball tank as an 800 psi filling station I think a small cartridge about as big as one of these would give viable power and the possibility of automatic ejection. Using the construction methods I used in my pengun v2 they should be easy to "mass" produce too.

...with mention of the NATO ammunition and all, got me thinking. Along the hybrid cartridge concept, what if you used a NATO cartridge? How do you get the fuel/oxy in there? Similar to a spool valve, the bullet would be used as the spool valve core. With the bullet 1/2 in the cartridge the valve would align, fully load the bullet and the valve would seal. Not sure the compression bullets have against there cartridge, but I suspect you'd get double digit mixes.

[10x Propane= 6.174 PSI]
[10x Oxygen= 30.723 PSI]
[Total=36.897 PSI]

I'm actually considering using a 7.62x51 cartridge, I have a couple of empties lying around not much success with a pressurised hybrid on a tiny scale from my part though

GOd it's hard to pump past 250 psi on the shock pump!!! and the volume it pumps it so small XD

Seriously? How big is your chamber then?

The chamber is a 3/4" tee, plus 3/4 => 1/2 => 3/8 => 1/4 " and a schrader valve epoxied in the 1/4 nipple.
I have an aching red-spot in my palm just pumping to 300 psi!!!

Mod it

We have the same pump, i'm gonna mod it XD frickin hard, and the tee had a small leak i fixed... BRB with some pictures and feedback, in another topic lol

jackssmirkingrevenge wrote:I'm actually considering using a 7.62x51 cartridge, I have a couple of empties lying around not much success with a pressurised hybrid on a tiny scale from my part though

To bad you are not in my area.
I could offer about 200 cartridges of .243 and .308 caliber.
Basically the same brass. The .243 is necked down.

Shame I'm wondering if they have enough volume for a 0.22 projectile though. They would certainly be ideal for a 3mm calibre cartridge but I'm not sure if going that low is worthwhile.