like tye speed of tue gas as it escapes through the gas port into the tube?or any idea how much faster it is than the bullet? I don't think anyone has probably ever ask this question specifically but the data might be there in some studies about Port pressure or something

Gas velocity in the gas tube?

Mach 1, it's a choked flow.  For that gas, at that temperature, around 1800 - 2000 fps.

It's the same velocity for all gas systems, regardless of actual port pressure.

The gas velocity in the barrel varies from zero in the chamber to the velocity of the bullet at the base of the bullet.

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Does constricted gas always flow at Mach 1 or something?

I'll expect the gas to want to travel faster than 2000 feet per second sense the difference between how fast it wants to go in how fast is able to go when is trapped behind the bullet is what makes the bullet accelerate. There are plenty of rifles that shoot bullets much faster than 2000 feet per second

Yes, choked flow has a maximum velocity of the speed of sound for that type of gas and the temperature.

A good primer on the subject: Mass flow choking

Choking has an advantage of also allowing the downstream pressure to be regulated, very useful in gas operated weapons.  For the M16, the pressure at the gas port is upwards of 12,000 psi, while the pressure in the cylinder peaks at around 2500 psi.

The reason why the gas velocity in the barrel is much faster is it is unrestricted as it expands.

pressure at the key for a 14.5" M4 is 125psi... 100psi for a 20" M16

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I read that link, for what good it did lol.  

So what if the tube was bigger?  What if instead of a .120 OD AR gas tube it was say... .215" ID?

Not asking you to tell me an exact figure but is the idea that the hot gasses would flow faster, being less restricted, or is that 2000 fps a hard limit regardless of the amount of restriction?

I was thinking about making a 16 inch M4'gry barrel and putting a series of ports in it starting at about 10.5" to spread pressure release so it doesnt all occur at uncorking, maybe venting it forward in gas tube or a larger SS tubing, perhaps with a few or many coils

I made a big mistake....

I based the velocity of sound on the incorrect temperature (F vs K).

The speed is considerably faster than 1800 fps, it is more along the lines of 4000 to 4500 fps

Than you sir...


That gives me something to think about knowing the gas would actually flow more than twice as fast as the bullet.