whats the best size container to compress air into?

Thanks for the advice on my earlier question guys but now I'm wondering what the best size to make my tank. Just so everyone knows I'm trying to improve a pneumatic  air soft gun I made a while back. I just want to know if a really small canister, like one with an aria of 12 square inches for instance, could launch my air soft bb just as fast as a 100 sq inch canister if they both had the same amount of pressure inside. If not then whats the best canister size in between the two to launch my air soft bb. Thanks for the help : )

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Toga_Dan2 months ago

for volume, it's cubic inches. Area is Sq inches.

If you are dumping the whole volume for 1 shot, then a big container doesn't help. It'll just end up being a lot more work to pressurize it with very_ minimal increase in power.

Toga_Dan2 months ago

For the math:

If the initial pressure in the tank is P1

And the volume in the tank is V1

And the volume of the barrel is V2

Then the final pressure as the projectile leaves the muzzle is P1x(V1/(V1+V2)).

inconceivable1 (author)  Toga_Dan2 months ago

ok so smaller is better cuz I do want to dump all the pressure at once. Got a size estimation?

if you share with us the dimensions of your tank and barrel, and do a bit of the math, I'll double check your numbers. What you really don't want is negative pressure by the time projectile exits.

inconceivable1 (author)  Toga_Dan2 months ago

ok i'll do my best but i don't have it with me any more. My barrel was a 45 cm long break pipe with the diameter of an air soft bullet. Before it was a reducer that turned the 1in pipe into a 1/2 in pipe so my barrel could be screwed in. Then there was a nipple that connected the reducer to a ball valve. on the other side of the ball valve was another reducer that connected the 1in diameter ball valve to to the 2 in chamber. the caber was 8 in long. Ok thats all I can remember thanks!

airsoft is 6mm IIRC.

Sounds to me like you had plenty of volume in the tank. As a guess, you had maybe 10x more air in the tank than in the barrel or more.

(i don't feel like converting exactly, but 45 cm is 17 inches, 6mm is about 1/4 inch, so.... approx 8x dia tank/dia barrel... = 64x cross section...... and barrel is 2x longer than tank... So.... yeah, you had about 30x more volume in tank than in barrel)

If you get annoyed with pumping a long time, then go to a smaller tank. if you think you can get more power, bigger tank. But theres a point of diminishing returns. You could probably increase your tank by 1000 times, and only get 1% more power.

Assuming that your projectile is a tight fit in the barrel, then a tank 3x the barrel volume is probably adequate in most cases.

inconceivable1 (author)  Toga_Dan2 months ago

Ohh! thats why it took so long to pump. Wow thats really helpful thanks, if it wasn't for you I would have probably just kept trying to make my tank bigger! So a small canister will give me the same power and a lot less work. Thanks so much for your help : )

Toga_Dan Toga_Dan2 months ago

when doing this math, sometimes an approximation is good enough. Also, it is the ratio of V1 to V2 that is important. So if you estimate that the tank has 10x the volume of the barrel, then you can plug "1" into the equation for V2, and "10" into the equation for V1. Then compare the difference in final pressure for a bigger air tank.

Downunder35m2 months ago

Take s small CO2 fire extinguisher or get a paintball canister.
They have the highest pressure rating you can get for a budget price.
Depending on your design skills you can even incorporate the trigger from the fire extinguisher into the firing trigger of the rifle ;)
Not the easiest task but possible, otherwise just seal it all of and add a pressure hose connector.
I don't know your design and how you plan to utilize the air pressure but check the many animations and drawings for paintball mechanisms through Google and Youtube.
I think opting for a single chamber with a spring loaded trigger that is forced back by the air pressure might be the best.
Unless you plan on something with a hinge or sideloader....

inconceivable1 (author)  Downunder35m2 months ago

ok sounds good whats thanks!

I think it would come down to weight vs volume vs pressure argument. Is it more important to be agile and have a balanced gun or are you going to camp and shoot lots of bbs. I would think that most air soft companies/players/message forums have this figured out to a few different size canisters. I would check around and see what the most popular sizes are for the type of shooting you are going to do and build it based off of that.

inconceivable1 (author)  cerberustugowar2 months ago

ok thanks