How much energy is bled off thru an AR gas tube?

drobs

July 06, 2012 00:33

Figure your average 55 grn bullet leaves the muzzle at 3240 fps creating 1282 foot pounds of energy.
http://billstclair.com/energy.html

How much of that energy travels through the gas tube and into the gas key on the bolt carrier?

Edit: See if I can clarify, looking for how much pressure is coming out of the gas tube into the bolt carrier.

There must be some way to measure the gas coming out of the gas port in the barrel and does that pressure decrease as it moves through the bends in the gas tube?

Comments

9 comments

v35

July 06, 2012 05:14
An easy estimation of loss would be to fire same load through a bolt action, preferably. One could use "Ballistics by the inch" tables
for longer barrels.
Other variables enter into the comparison making it a rough estimate.
0
beantownshootah

July 06, 2012 06:05
You want to know in actual foot pounds?

V35 has it. Compare kinetic energy of a round from an AR-15 to the same load fired from a bolt-action rifle with the same barrel length.

I don't have data here, but I know its not much energy loss, only a few percent at MOST.

If you want some small basis for comparison, imagine how much energy it takes for you to cycle the action manually. Only a little bit more than that is bled off per round to do it semi-automatically.

Responding to below: quote:Mobuck
You can't compare velocities "from a bolt action with same length barrel". The difference between barrels is very likely more than the loss from the gas system. What if your comparison resulted in the AR having a higher velocity than the "similar length" bolt action rifle barrel?

I think its pretty unlikely that you'd find a bolt action to be slower than an AR-15, but if you did, I think it would be fair to conclude that the energy lost to gas bleedoff in operating the AR15 action is negligible. Actually, I think you can conclude that anyway. . .

Thinking about what I wrote above, as a sort of rough guesstimate, to cycle the action of an AR-15 manually takes maybe 10 lbs force over 6 inches distance. If anyone wants to actually measure this, its pretty easy. But for the precision necessary for this particular question, you're really only talking maybe 10 foot pounds of energy necessary to cycle the gun. Given that typical muzzle energy from an AR-15 is about 1300 ft-lbs, total energy loss from the gas system is going to be in the vicinity of 1% or less of total energy.

In terms of gas PRESSURE at the gas port, that's a different question altogether.

The answer depends on how far the gas port is from the chamber, itself dependent on what AR variation gun you're talking about.

For obvious reasons, the closer the gas port is to the breech of the gun, the higher the peak port pressure.

Exact numbers will vary from gun to gun and depending on ammo, but for a full sized rifle, pressure at the gas port is about 20k PSI give or take. For a carbine, its about 30k. Mid-length systems are in between. And AR-15 pistols with really short gas systems can see pressures over 40k psi.
0
Hawk Carse

July 06, 2012 06:54
The takeoff of gas to the action is pretty small relative even to the difference between two barrels. Firing a bolt action beside the auto would not likely tell much.

The real way to measure it would be to get one of those adjustable gas blocks and shoot it with the port open and closed in THE SAME barrel. Be prepared to do a lot of shooting to build up some statistics; the shot to shot difference of the ammuntion will show up strongly, too.
0
Txs

July 06, 2012 07:49
An insignificant amount.

It varies based upon a few factors, but that gas port is in the neighborhood of just .06-.09 in diameter and the pressure remains contained within the gas tube and bolt carrier as the bullet passes down the remainder of the bore.

The bullet has left the muzzle by the time this pressure is actually released to the atmosphere through the side of the bolt carrier.
0
Mobuck

July 06, 2012 08:09
You can't compare velocities "from a bolt action with same length barrel". The difference between barrels is very likely more than the loss from the gas system. What if your comparison resulted in the AR having a higher velocity than the "similar length" bolt action rifle barrel?
0
62fuelie

July 06, 2012 14:21
The loss would not exceed the standard deviation between rounds of the same lot. The tiny bleed hole and the short time the gas column is working against it before the bullet exits the barrel would not be statistically significant.
0
Txs

July 07, 2012 12:15
quote:Originally posted by drobs
Edit: See if I can clarify, looking for how much pressure is coming out of the gas tube into the bolt carrier.

There must be some way to measure the gas coming out of the gas port in the barrel and does that pressure decrease as it moves through the bends in the gas tube?
Only slightly.

IIRC, it's around 50-55,000psi at the carrier key.
0
SoreShoulder

July 07, 2012 13:05
quote:Originally posted by Txs
quote:Originally posted by drobs
Edit: See if I can clarify, looking for how much pressure is coming out of the gas tube into the bolt carrier.

There must be some way to measure the gas coming out of the gas port in the barrel and does that pressure decrease as it moves through the bends in the gas tube?
Only slightly.

IIRC, it's around 50-55,000psi at the carrier key.
That's close to the peak pressure inside the barrel. It only exists near the beginning of the bullet travel. I would say the port pressure can't be much higher than 20,000psi in a standard length barrel, then the port throttles it down severely. The bullet is probably doing well over 2500fps as it passes the port so the port is only exposed to pressure for the shortest time.
0
sandwarrior

July 08, 2012 03:42
drobs,

Here's what I've seen:

On a pretty much across the board observation I see bolt guns getting a smidge more velocity using equal length barrels. It's not the length of the barrel in an AR, it's the length of the gas tube. I tend to see bolt guns gaining in relative effectiveness in comparison to their AR counter-parts as the barrels get shorter.

Some things to remember about the operation of the AR rifle. When the cartridge fires, it's locked up just like a bolt-gun. There is no loss of energy/velocity until the bullet passes the gas port. From the initial ignition the cartridge has produced from zero to 55k+ psi of gas and is now down near about 17k psi as the bullet passes the port in a standard length rifle. After the bullet passes the port, there is a relatively huge area of expansion to fill in the gas tube. Gas pressure is dropped to about half what it was before the bullet passed that point.
In progressively shorter rifles you will have less time for the gas to push the bullet and the gas also begins being bled off from a higher pressure. Which equates to a higher percentage of loss in the AR vs. the bolt gun.
The way to test this, is fire enough rounds from the AR and the bolt gun that you can get a mean average of how they both shoot and specifically how the ammunition shoots. This includes all full length rifles and all susequent short rifles. Then compare how the bolt guns fare against the AR's and see if this works for you.

FWIW, if you do as Hawk Carse said and install an adjustable gas tube and see how that works. Go from standard to closed (simulating a test between your AR and a bolt gun) and then progressively from standard to "open" and see how much velocity is lost (simulating shorter barreled rifles.)
0