SpudFiles

1/4 Diameter will give a port area = to the cross section of the barrel. This gives in essence two restrictions choking flow of the same size, the barrel area and the port area from the chamber through the gap into the barrel area. I prefer to use a port area that is about twice the cross sectional area of the barrel so the velocity of air entering the area behind the projectile is traveling 1/2 the velocity as the air entering the barrel for lower pressure drop on the first restriction to flow. This leaves the barrel choking the flow as the only considerable limitation to flow.

Don't get me wrong, it is still great flow.

1/4 diameter does provide the same cross sectional area as the barrel, but it is not without turbulence and some pressure loss.
This great graphic showing the pressure between the piston and barrel with a wide piston gap shows the area between the piston and barrel is essentially at chamber pressure.

Technician1002 wrote:1/4 Diameter will give a port area = to the cross section of the barrel. This gives in essence two restrictions choking flow of the same size, the barrel area and the port area from the chamber through the gap into the barrel area. I prefer to use a port area that is about twice the cross sectional area of the barrel so the velocity of air entering the area behind the projectile is traveling 1/2 the velocity as the air entering the barrel for lower pressure drop on the first restriction to flow. This leaves the barrel choking the flow as the only considerable limitation to flow.

Don't get me wrong, it is still great flow.

1/4 diameter does provide the same cross sectional area as the barrel, but it is not without turbulence and some pressure loss.
This great graphic showing the pressure between the piston and barrel with a wide piston gap shows the area between the piston and barrel is essentially at chamber pressure.

It certainly is like that in a coaxial, but my piston will be housed in a tee, so flow might be restricted a bit.
What program are you using in your simulations? (would like to do some research too)

Anyway I think I will be building a hybrid valve (half standard piston half QDV ) (not cannon) or a QDV