Making a CO2 powered BB gun

[trains are bad]

I was thinking about making a BB gun that took a CO2 tank from a paintball gun, and paintball parts. But then I realized that since BBs are so much smaller in diameter than paintballs, then there might not be enough pressure to get good velocity. I calculated the difference in the area/mass ratio for paintballs is about 1.5 times what it is for a BB, so to a first approximation I can expect to need 1.5 times the pressure to accelerate a BB to the same velocity as a paintball. Paintball markers run as low as 2-300PSI, but only shoot 300fps. If I wanted say 650FPS I would need 900PSI which is all you can get from raw CO2. I know the real powerful pellet guns use a spring piston. I know there are CO2 powered BB guns, but I think they only get 700fps at the most out of using raw CO2, and that's with the long barrel. But then I'm not sure if they intentionally throttle using valve dwell in order to get more shots/cartridge. I'm wondering if I used the highest-flow valves etc that I could, how much velocity could I get out of raw CO2 with a BB?

It's interesting that using larger BBs doesn't help, because volume scales as R^3 but area scales as R^2.

[firsttarget]

I would look at this barrel first before doing any major work:
http://www.barragebarrel.com/index.php?main_page=product_info&cPath=67&products_id=180&zenid=8e7ca6ad1775e7269ab2744a6011c6f8

[trains are bad]

Barrage Barrel™ will fire at an average 360fps for steel BBs

Lame. My goals are higher.

It is, however, good information that you can get 360FPS with a 10-inch barrel. Bolted straight onto an unmodified paintball marker. So you could use paintball parts to at least work. I bet if you optimized paintball parts, such as a rocket valve at high pressures, and a reasonable barrel, you might get satisfactory velocity.

[Hilltop Customs]

making one from scratch or building one off of a existing bb gun?

I was working on converting a pump to constant air....I planning on tapping the pressure chamber then using a guage to determine the pressure after # of pumps and corresponding velocity of the shot. Either way the chambers going to have tome input port of air/co2, so its not really a waste trying it like that. Then again, I was planning on using regged HPA. More than likely the chambers walls will be too thin for actual tapping, so it may be easier to replace the pump plunger with a seal with integrated NPT fitting, but that makes figuring out the pressure a PITA.

Never got around to finishing it....had too big of plans for it, conversion from bolt action to semi using cocker pneus, spring fed bb magazine, and constant air. Got the semi action working fairly well, but royally screwed up the spring fed magazine and just got tired of it.

[trains are bad]

if you have a chronograph you could always try slowly increasing the pressure until the gun measures the same velocity as X number of pumps does.

I was going to build mine from scratch. I don't have machining capabilities so it's strictly in the design phase right now.

[gommie404]

co2 wont get the psi you need in 4.5 cal. your have to go to 22 or 5.5.mm
Or for you chaps over the pond make use of that 20 cal.5.00 mm

But i do like the look of that bb paintball barrel.
i think i have just found my new rat blaster.

I have in the past mixed a lot of air guns with paint markers .
And the co2 wont make the psi you want in 177.

but your still get a good can whacker and it will still deal with small vermin.

happy hunting