Cut barrel debate... 18 to 14.5in

[ar15barrels]

Well I did mention that the port could be different (reduced or expanded) to compensate for the length (shorter or longer) past it depending on the manufacturer. But I was running under the impression that gas pressure inside the barrel at the port when the bullet reaches it would be the same if all else were equal.

The pressure at the gas port is the same regardless of how much barrel is past the gas port.
It takes TWO things to make the gun cycle though.
Pressure AND Duration.

The amount of barrel past the gas port primarily effects Duration.
The gas size port effects how much Pressure can build inside the action during the specified Duration.

[Thorax]

Pressure and VOLUME.
The amount of volume is more important than pressure.

If you have 50,000 psi and only 1 cubic millimeter of volume, the bolt will probably not even unlock.
If you have 100 psi of pressure but 30 cubic centimeters of volume the bolt will probably blast out the back of the action ruining everything including the user.

Simple hydraulic displacement theory.

[ar15barrels]

Pressure and VOLUME.
The amount of volume is more important than pressure.

If you have 50,000 psi and only 1 cubic millimeter of volume, the bolt will probably not even unlock.
If you have 100 psi of pressure but 30 cubic centimeters of volume the bolt will probably blast out the back of the action ruining everything including the user.

Simple hydraulic displacement theory.

Your Volume variable is effecting the surface area that the pressure is acting on.
You can have 1 milliliter of volume and 30 cubic centimeters of volume and have the same force created if both volumes are of the same ram diameter.
The 1 milliliter volume will just be MUCH shorter.

The surface area inside an AR bolt carrier is fixed as they are all the same size if we are staying within the same AR familiy size.
The variables in tuning the AR direct impingement system are the gas port location (supply pressure), the gas port size (flow limiter) and the duration of flow (amount of barrel past gas port) and the mass of the carrier/buffer.

[Thorax]

ssccchhheeeeewwwww!

That was the sound of engineering flying over your head Randall.

Volume is a constant, the size of area affected also affects the volume.
Your worrying about time (duration) is of zero consequence.

Given a certain orifice size and a certain pressure over a duration that the pressure flows, creates a given volume.

Repeat, a given volume that is constant.

I could work up a whole mathematical formula for you given a bit of time that I just don't feel like giving right now....but it's not that tough to math out, really.
Maybe it's just too frikkin cold....dunno, it's sub freakin nothing with a wind chill of absolute zero out there right now :/

I'll try and simplify it for you....
Say you have a 22-250 and it puts out 60k psi.
Plenty of pressure, right ?
How well will it move a 750 grain 50 cal amax bullet ?
There's a volume limitation....the case doesn't have the capacity to provide the volume of powder needed to provide the volume of expanding air that moves the bullet.

Gads, I suxorz at 'splainin the engineering stuff.

[ar15barrels]

Your worrying about time (duration) is of zero consequence.

In your hyrdaulics comparison, that's correct.

In AR's the duration is so short that the gas system never even fully pressurizes before the supply pressure in the barrel dissappears out the muzzle.
In AR's duration matters greatly as we are timing the duration of flow to move the carrier before the bullet leaves the barrel.
Carrier movement prior to bolt unlocking vents the pressure that's moving the carrier so that only the stored energy in the carrier cycles the carrier rearward after the venting.

[Thorax]

ok, I'll slow this down a tad.

Envision a air line with a quarter turn valve on it connected to a air compressor.
If you open the valve for a small time you get a small volume of air at whatever pressure the compressor is set to.
If you open it for a longer period of time you get a greater volume.
Obviously, right ?

A bullet travelling down a AR barrel acts exactly like a valve in the above.
It both opens and closes the valve (pressure exposure to the gas hole).

We'll use the standard 3250 fps of a milspec 55 grain load.
(hey, it's what they claim, right ?)
It should run about 60k psi.

The velocity of the bullet sets the time from just passing the gas hole to the muzzle (valve open time).
The pressure of the explosion gives us a ~reasonable pressure wave to work with.

The pressure is not constant and is degradational over the period of time the bullet is in the bore (expansion losses behind the bullet, etc).

There is dead air in the gas tube that must be accounted for as this is what creates the initial compression.

Gahh....i'll pick this up tomorrow, too cold, too tired.

[ar15barrels]

ok, I'll slow this down a tad.

Envision a air line with a quarter turn valve on it connected to a air compressor.
If you open the valve for a small time you get a small volume of air at whatever pressure the compressor is set to.
If you open it for a longer period of time you get a greater volume.
Obviously, right ?

A bullet travelling down a AR barrel acts exactly like a valve in the above.
It both opens and closes the valve (pressure exposure to the gas hole).

We'll use the standard 3250 fps of a milspec 55 grain load.
(hey, it's what they claim, right ?)
It should run about 60k psi.

The velocity of the bullet sets the time from just passing the gas hole to the muzzle (valve open time).
The pressure of the explosion gives us a ~reasonable pressure wave to work with.

The pressure is not constant and is degradational over the period of time the bullet is in the bore (expansion losses behind the bullet, etc).

There is dead air in the gas tube that must be accounted for as this is what creates the initial compression.

Gahh....i'll pick this up tomorrow, too cold, too tired.

We are probably saying the same thing, just with different terms.
I'm tired too.