OK, finally found time to work on the rifle a bit. So here's what happened :

I fixed a lil' design problem with the trigger and put it together, even completed the breech loader sys.
And then I got myself a car pump and it was time 4 testing...
But freakin' surprize when I herd the devastating hissing sound... yes... the valve leaked at exactly 40 PSI. Damn !
The problem is that I used the valve sys backwards (willingly, of course, so I can read the pressure and I can use the safety valve). Well, I figured that if it works one way, it should work both ways just as fine. But I was wrong.
So I spent the rest of the day figuring out how to use the sys without losing the ability to read the pressure off the gauge and to let the air out in case of emergency.
And imagination prevailed again... I had to use TWO such "valve guns",back 2 back, and that way I also solved the air tank problem.
The only problem now is that it really doesn't look like a rifle, so I need to think about integrating it into an appealing design fit for combat conditions.

You put it on top of the gun...
That IS the basic idea, isn't it ?

I do believe I know my own words and I believe I said "I bet some of the guyz here don't even know Pascal's law. "

And it's an elementary law, one that all of us take for granted, one without which nerfing, hidraulics, and God knows what else wouldn't exist.

But 'nuff of that. I just don't see what the big deal is. I mean... the pressure delta 'tween zero and 90 degrees F can be overlooked, it can be compensated by one or two more pumps. It's not like we're building Feinwerkbau grade airguns... it's freakin' foam...
Don't mind me, I guess I'm just too Eastern-European, doin' it the Russian way: build first and calculate later. ( And I wonder why there are more AK47s in the world than M4s ).

PS: I'm not a big fan of Automatik Kalashnikov.

5 to 1 ?! Man... Have you seen the episode in Scrapheap Challenge where they built huge air cannons with about 1.5 to 1 chamber to barrel volume ratio (the barrels were like one foot in diameter). And I've heard professionals talking about 1 to 1 ratio.
The thing is you can't just settle on a ratio. There are just too many variables, like you would wanna build up a lot of volume if you use an automatic gun (so you won't have to reload often) but if you have a single shot you might wanna use a small air chamber with lots of pressure (that way you don't have to work a sweat pumping).

PS: Don't anyone even think about calling me a newb, 'cause I've been in this field for like 4 or 5 years and I bet some of the guyz here don't even know Pascal's law.

if you are using mostly steel tubing that you should be able to hold plenty of air pressure.

I doubt a 0.3 mm thick steel sheet can "hold plenty of air pressure" (I think that's about the thickness of the walls of a bug spray can).
Well, that's the thing : I've settled on 150 PSI ( 10.5 bar ) and I don't even think an average car tire pump can take that much ( maybe I'm just skeptical 'cause the M8F5 blew up in my face once and that was only 4.2 bar ). The thing is I've blown serious firecrackers in aluminum pipes, made plastic and cardboard flamethrowers and now I only wanna make something a lil' safer for me...
Haven't had the time to work on the rifle this week and I don't think I will any time soon. But I've one idea for a small air chamber so things are looking up. Stay tuned.

OK, 10x for the tip, but I REALLY don't trust PVC for these kinda jobs ('specially 'cause I don't have and can't get any good solvent/glue/cement for it) so it's steel all the way for me.
I think I won't even need an air chamber since the volume in the barell is really small... well, I think I'll get on it this weekend. Keep in touch !

A little intro :
I've liked weapons since I was a lil' kid, first got a stupid BB gun, modded it to a cool one, then made two or three BB rifles of my own, then made the Railgun, then the M8 N1 (a real freakin' rifle, with bullets and all...), then the M8 F5 (powered by bug spray : both rifle and flamethrower - not very powerful)... well, it's all here : http://fiveseven.xon...n.com/Guns.html

So now I'm thinking air power...
I'm posting my work here 'cause it's pretty related to wargames, even tho' it's a lil' bit more serious and more dangerous than backyard nerfing...
Plus, it's definately related to airguns and you guyz are all about homemade airgunz (rock on !).

OK, let's get technical.
The caliber is 4.5 mm (Magnum)
Barrel lenght around 450 mm
Valve is the kind mechanics use to pump up car tires (freakin' great 4 this kinda job)
The air sys can take up to 18 bar (around 180 PSI ,I think)
I'm thinking usual pressure of 150 PSI

The thing is I don't have an air chamber... I'm thinking about using a bug spray-hair spray can but I don't trust it to hold 150 PSI. So if anyone can give me an idea, I'd appreciate it.

I'm not sure about the design, either. So I'll get back to you ASAP with skeches or something.

So... any thoughts on the chamber are welcome, plus I'm considering not using a chamber but using a constantly pressed car pump instead (hope I have enough air) that would be nice 'cause I wouldn't need valves and all that... trying to keep the design to a minimum...