AR15.Com Archives
 How much pressure does an AR need to cycle?
Capital  [Member]
9/9/2010 3:21:23 PM
An irrelevant question but Ive been wondering how much pressure the gas system in an AR sees. I looked and couldn't find any figures. Judging by the resistance of the buffer spring my guess is around 5psi but I could be way off.

Anyone know?
Paid Advertisement
--
Captains1911  [Member]
9/9/2010 3:25:48 PM
Originally Posted By Capital:
An irrelevant question but Ive been wondering how much pressure the gas system in an AR sees. I looked and couldn't find any figures. Judging by the resistance of the buffer spring my guess is around 5psi but I could be way off.

Anyone know?


5psi, really? You do realize that chamber pressures are upwards of 65,000 psi.
WMurderface  [Team Member]
9/9/2010 3:27:24 PM
There is a chart floating around here with different pressures for different gas systems. I don't have it, but I think I can say that 5psi is WAY off

Posted Via AR15.Com Mobile
CONKLE73  [Team Member]
9/9/2010 3:35:28 PM
Russ4777  [Team Member]
9/9/2010 3:41:57 PM


I agree that 5k psi is way off for any AR.

Nice chart. What is the source of this information? It does look believeable. I had previously heard that the peak gas system pressure in a full-length rifle was around 20K psi. This chart confirms that value.
Fields_Overseer  [Member]
9/9/2010 3:57:41 PM
IDK, To find out on paper, you need to find out how much force is required to cycle the action. For the force from the gas, you have to realize that the higher the pressure, the shorter the time frame for the action to absorbing energy. This would probably require a differential equation. Then you have to account for leaks(action is not air tight). Then set up some equations to find out the min PSI.

Short dwell times require high pressure and vise versa. This is why 11.5" carbines(35k psi) only have 2.5" in front of gas block while rifles(20kpsi) have about 6" in front of the block. They both deliver about the same energy to the action though.
Capital  [Member]
9/9/2010 3:57:56 PM
Missed the "k" after my 5 but still I was way off. I had no idea it saw those levels. I realize the chamber pressure is very high but wasn't clear on how much pressure the gas tube saw. If you were to meassure pressure where the gas exits the tube and enters the gas key would it still be this high?

Thanks for the replies to my noob question. Very interesting stuff.
Captains1911  [Member]
9/9/2010 4:01:54 PM
Originally Posted By Capital:
Missed the "k" after my 5 but still I was way off. I had no idea it saw those levels. I realize the chamber pressure is very high but wasn't clear on how much pressure the gas tube saw. If you were to meassure pressure where the gas exits the tube and enters the gas key would it still be this high?

Thanks for the replies to my noon question. Very interesting stuff.


The pressure immediately begins to decrease as it leaves the chamber, and is no where near that high at the carrier key. That is one of the reasons for the gas operated system, it allows the pressure to bleed off to a safe level prior to it reaching the ejection port. Gas port pressure and pressure at the bolt carrier key depend primarily on barrel length, gas system length, and ammo.
Forest  [Moderator]
9/9/2010 4:18:03 PM
Originally Posted By Fields_Overseer:
.They both deliver about the same energy to the action though.


No they don't.

Carbine gas system are notoriously harder thyan rifle gas systems on their parts. The pressure is higher and that translater to faster bolt carrier velocities.
Fields_Overseer  [Member]
9/9/2010 4:43:48 PM
Originally Posted By Forest:
Originally Posted By Fields_Overseer:
.They both deliver about the same energy to the action though.


No they don't.

Carbine gas system are notoriously harder thyan rifle gas systems on their parts. The pressure is higher and that translater to faster bolt carrier velocities.


Edit, not if the dwell time is short enough. Assume the force of the BCG is in newtons. Thats (kg*m)/sec. Pressure is in (kg*)/(m*sec^2). If the pressure is higher, for the same force, the time is lower, which the 11.5" has. That means you have an infinite number of pressure/dwell time combinations that have the same force. Just because the pressure is higher does not mean there is more force. Not saying they are the same, but close..

i said about. Meaning its not exact, but close. Its enough to reliably cycle the action, but not too fast either. 2nd, the rifle has a heavier buffer. If you put the same energy into 2 object, the lighter one moves faster. When using a heavier carbine buffer the speed slows down. I read that(dont have a scale) rifle buffers are 5.something and carbine buffers are 3 oz. Stick an h3 or 9mm buffer in a carbine and whats the speed?

also, they are VERY rough on 14.5" m4's but not as much on the 11.5" barrels which i was talking about.

Anyway, the point was that your increasing pressure by 50%, but since the dwell time is so much shorter, it gets you to the same(roughly) energy.

It would be interesting to get a high speed camera and compare the speed difference of the action on a rifle buffer system between a 20" rifle and an 11.5" carbine...
Fields_Overseer  [Member]
9/10/2010 2:37:31 AM
Originally Posted By Capital:
Missed the "k" after my 5 but still I was way off. I had no idea it saw those levels. I realize the chamber pressure is very high but wasn't clear on how much pressure the gas tube saw. If you were to meassure pressure where the gas exits the tube and enters the gas key would it still be this high?

Thanks for the replies to my noob question. Very interesting stuff.


No. For one, the gas block will reduce pressure. Also, the small diameter of the tube will also reduce the pressure or dwell time of the pressure on the action. Think of sucking a drink through a straw, it doesnt instantaneously enter your mouth. The gas needs time to equalize throughout the system. I have no idea on the numbers. For all intends and purposes it might be "instantaneous". I dont feel like doing all the work right now to actually find it. Its like one of the reactor design questions we would have for homework that takes a couple hours to figure out.
Paid Advertisement
--