Gas Systems+Pressure+Barrel Length

[chrisward3]

Ive been reading through threads for awhile now, and have never quite understood the relationship between gas systems, barrel length and pressures.

I understand that a bullet fired out of a shorter barrel will not go as far as a longer one, as the powder has more time to accelerate the bullet down a longer barrel.

The issue im confused on is that people say that a 10.5" AR will have a lot more pressure returning to cycle the bolt/action than say a 16".

In my opinion, if both rifles have a carbine length gas system, shouldnt the pressure of the gases returning to the chamber be the same?

The bullet is still travelling the same distance, with the same amount of pressure behind it at the point you hit the gas port (10.5 or 16), so why is it that people say a 10.5" has a lot more pressure than a longer rifle?

Opinions, ideas? or im just out to lunch?...lol

cheers

 

[greentips]

due to the relative small barrel infront of the gas block, the gas port of a short barrel rifle needs to be bigger as the amount of time of pressurization within the system is shorter.

If you think of the total amount of energy require to activate the system as a min. threshold, a short barrel system needs to have higher pressure ( larger gas port with larger flow area, since gas velocity is the same irregardless at certain point of the barrel at certain point of time) with less amount of time of pressurization.

 

[paulz]

What Greentips said....One of the things you must remember is that as soon as the bullet leaves the barrel, pressure drops very rapidly/almost instantaneously to ambient pressure, so the distance the bullet travels AFTER it passes the gas port is important, since the system "depressurizes" as soon as the bullet leaves the muzzle.