GGDT strange behaviour

[Demon]

Some of you may know, but I want to change the caliber of my M82A1 project to avoid lawsuits, to convert it to cal .69 (paintball) instead of cal .545 (cartridges and wadding).

But, as I haven't expected at all, I got much worse performances with the paintballs, altought they weight less and have a bigger surface area.

Fixed parameters:
Chamber volume, air pressure, barrel length, valve size.

Variables :
Barrel diameter.

I barely get 400 FPS with a 3.2 gram paintball in .69 barrel diameter, while the 4 gram .545 projectile was going way ofer 500 FPS.

What could cause that ?

[Crna Legija]

how big is the chamber, you should go for 1:1 if your using under 250psi

screen shot ggdt would be nice.

[warhead052]

Weight. And possibly shape? Maybe because the paint inside the ball is moving around as it flies, causing it to mess with the speed? Try making a rubber round, the same size.

[Technician1002]

Limited volume of air, and limited rate air can fill the new larger volume of the barrel.

I would vote valve first, and chamber second.

[jackssmirkingrevenge]

What could cause that ?

The valve is feeding x amount of air into the barrel.

If you increase the barrel diameter, yes you've increased the cross-sectional area of the projectile for the pressure to act on, however you've also proportionally increased barrel volume, meaning pressure will rise at a lower rate.

Also, heavier projectiles compensate for lower valve speeds and flows, so muzzle energy tends to be higher, though not necessarily muzzle velocity.

Weight. And possibly shape? Maybe because the paint inside the ball is moving around as it flies, causing it to mess with the speed? Try making a rubber round, the same size.

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[Demon]

Jack and technician, it seems you are right.

Simply making the valve diameter bigger of one millimeter gives tens of m/s more.

(And I can't make the valve seat any bigger)

So that means that the efficiency of this gun would go way down if I made the barrel bigger ? Where does all the pumped in energy goes ?

[jackssmirkingrevenge]

So that means that the efficiency of this gun would go way down if I made the barrel bigger ? Where does all the pumped in energy goes ?

It's wasted if you use a light projectile, try modelling some heavier rounds with the larger barrel and you should see a rise in efficiency and muzzle energy.

[Technician1002]

Where does all the pumped in energy goes ?

Same place the high HP from a shop air compressor goes when spray painting. The energy goes into making very turbulent air. You are spraying air into the barrel, and the valve is limiting the volume delivered.

A larger valve permits delivery of a higher volume of air into the barrel with less pressure drop on the valve and more pressure on the projectile instead.

I try to build valves to just get out of the way of the flow and do it quickly.

For high speed and light projectiles, the valve is the big limiter.