The ideal gas law is not usable in the situation of a pneumatic spud gun. Three state variables (pressure, volume and temperature)n al change. You need to do more complex stuff, like take in to account the heat capacity of the air. There are very nice tools for both pneumatics and combustion guns de...

The biggest I usually see is 150 ml. That's big enough for 1x in a 3.75 liter (229 in^3) chamber, which is a pretty big chamber. https://www.amazon.com/Syringes-Scientific-Measuring-Watering-Refilling/dp/B07C71C1LH/ref=sxin_7_ac_d_rm?ac_md=2-2-bGFyZ2Ugc3lyaW5nZQ%3D%3D-ac_d_rm&cv_ct_cx=syringe&dchild...

Moonbogg wrote: ↑

Sat Jun 06, 2020 2:07 pm

jackssmirkingrevenge wrote: ↑

Sat Jun 06, 2020 6:05 am

jimmy101 wrote: ↑

Thu Jun 04, 2020 10:31 pm

Caliper in the fruit isle? Just take a little length of barrel!

"Security? Yeah it's that weirdo putting limes through a pipe again..."

Yeah, that wouldn't look good.

Meh. Put a cut in the 1/2" long piece of pipe and wear it into the store as an ear ring.

Easiest way to understand C:B ratio is that if the barrel is too short, the projectile is still accelerating when it leaves the barrel (sub optimal), and if the barrel is too long, NEGATIVE pressure behind the projectile will slow it down before it exits the barrel ( if your barrel is very long, an...

Looks outstanding. But the coax design is killing the performance. I would expect a gun that size to be firing something like a spud at about 150-200 MPH. From the video I would guesstimate about 50 MPH or less. Any idea what the chamber and barrel volumes are?

But it does looks sweet.

The importance of the C:B ration is kind of over rated. If you look at the data, the velocity goes up as the chamber goes up for a fixed size barrel. Eventually the chamber get too big but that is up near C:B ratios of >5:1 or something. So bigger is generally better. The 0.8 value is actually more ...

"Burnt Latke", ah, those were the Days... Don't need a vacuum pump to run pure O2. Just flush the chamber with the O2 before adding fuel, not all that hard really, but might get expensive. But pure O2 is up around 5x the power of propane+air. None the less, it looks like you know what you are doing ...

What Jack said, but with a chamber that big a syringe is going to have to be huge. Is that a meter pipe system? if it is and you think the performance is lacking I would look at 2 things. (1) Is you mixture correct? Your fuel volume using air at 1 ATM is 4% of the chamber volume. (2) Big long chambe...

Caliper in the fruit isle? Just take a little length of barrel!

The teacher isn't a "professor", it is a high school teacher so expertise in this area is probably limited. :) Mark.f gave a reasonable answer. Don't need much of a "computer", you can do it an Excel spread sheet. Wont be super accurate but were talking a high school project, not a DARPA project. Ju...

You can search these forums for "honking" for a number of posts about the problem.
There is also a brief page on the wiki at spud_wiki/index.php?title=Honking and spud_wiki/index.php?title=Sprinkler_valve_modification

That is TRUE, in 35 years I never installed a PVC gas system. I have installed Polypropylene. PVC is not used on gas because it is FLAMABLE. SCH.40 PVC IS pressure rated to about 220lbs and SCH 80 to about 320lbs. OF course the Explosion of Propane probably creates more than those pressures, which ...

You could also just use a neodymium magnet if you are just trying to keep the BBs from rolling out'a the barrel.

Another challenge is that the chamber pressure probably oscillates a fair amount (pressure goes up when a bb is actually in the barrel, down when the barrel is empty). One approach might be to start with ROF and muzzle velocities for a BBMG some has already built, use that to calibrate GGDT, then mo...

Welcome to the forum!

Post some pics and specs, people are usually glad to help (or at least chime in with criticisms )