Question about specific gas port pressures

M

Metal god

New member
I was talking with someone and they mentioned that even though there is a possible 62k psi max pressure in the 5.56 AR barrel . As the bullet passes the gas port the pressure would be more like 10k to 15k psi in a standard 5.56 load .

First does pressure drop that much in the barrel ? I knew your peak pressure was only a few inches down the bore but never thought the pressure dropped that much by the time the bullet left the muzzle .

Now I know there are a few variables in that equation so there is no straight answer but lets give a specific load and guess ( or if someone can run a quickloads calc ) what the pressure would be

Rifles : lets do two with gas ports 12" down the bore and 7" down the bore

1) standard M16A2 with 20" barrel and rifle length gas system
2) M4 14.5" barrel with carbine gas system

Load :

55gr FMJ-BT
26gr H-335
LC-12 case
CCI #41 primer
COAL 2.240

Any thoughts ?

Me ? I really have no idea but my first thought would have been 40k psi at the gas port but now I'm not sure .
 
Some quick QuickLOAD numbers:

223 rem., hornady 55 fmj,

peak chamber pressure: 58,147 psi
7": ~28,257 psi
12": ~17,379 psi

standard disclaimer: QL estimates pressure and the pressure curve might not be accurate at those distances.
 
I know in an M1 Garand you're supposed to use ammo with a peak pressure around 50,000 PSI and gas port pressure of around 6,000 to 8,000 PSI. So yes the pressure does drop very quickly

As the bullet travels down the bore the change in air space is substantial. Pressure is directly related to volume. Everything else being equal you will have half the pressure at 4" compared to the pressure at 2", and at 8" down the barrel you'll have half the pressure you had at 4". Of course this is over simplistic because there will still be some powder burning and creating gas at 4" and 8", but most of the powder is burned within the first couple of inches.
 
Thanks 74A96 . That's the numbers I was looking for . Wow from 7" to 12" it drops 11k psi . Interesting , that would have never been my guess .

The M1 Garand numbers are interesting as well .
 
I have a LYmans 48 that was a yard sale buy (my wife not me)

Has some great information in it including that sort of data.
 
The Garand gas port is 22.5" down the barrel, so its gas port is at very close to muzzle pressure.

Expanding gas drops both in volume and in temperature simultaneously, so you can get rough estimates from the ideal gas law based on volume and temperature drop proportional to the increase in volume. Assume the peak happens when the bullet base is about 1" down the rifling as a starting point, then use bore and as-fired case water capacity to work out your volumes.

Look up military load specs for validation.

From MIL-C-46931F(AR), Cartridge 7.62mm, Ball, M80

3.7.1.1 Chamber pressure measurement at 70°F by copper-crush
cylinder test method. The average chamber pressure of the
sample cartridges conditioned at 70°F shall not exceed 50,000
pounds per square inch (psi). The average chamber pressure plus
three standard deviations of chamber pressure shall not exceed
55,000 psi. The chamber pressure of an individual sample
cartridge shall not exceed 55,000 psi.

3.8 Port pressure. The port pressure measurement can be
performed by either the copper-crush cylinder or EPVAT test method.
The average port pressure of the sample cartridges when
conditioned for not less than 2 hours at 68° to 720F and fired at .
that temperature shall be as noted in the applicable
sub-paragraphs:

3.8.1 Measurement by copper-crush cylinder test method. The
average port pressure of the sample cartridges shall be 12,500 psi
± 2,000 psi.​


From MIL-C-63989A(AR), Cartridge 5.56MM, Ball, M855

3.7 Chamber pressure. The average chamber pressure of the
sample cartridges, conditioned at 70°±2°F shall not exceed
55,000 psi. Neither the chamber pressure of an individual sample
test cartridge or the average chamber pressure plus three standard
deviations of chamber pressure shall exceed 61,000 psi.

3.8 Port pressure. The mean port pressure minus three
standard deviations shall not be less than 13,000 psi for sample
cartridges conditioned to 70°±2°F.

QuickLOAD graphs follow measured numbers very well. Here's one for .308 Win Firing 168 grain bullets. Note the bullet travel in inches at the bottom is for the base of the bullet, which starts out 1.585" forward of the breech, so that even though the barrel is 24", bullet travel is 22.415". The load is Hodgdon't maximum load.

attachment.php
 

Attachments

  • Varget Load 2017-12-05_12-53-03.gif
    Varget Load 2017-12-05_12-53-03.gif
    53.7 KB · Views: 316
Determining PORT Pressure will be nearly impossible without actual direct readings.
I know there will be a TON of speculation, none of which will be accurate.

The normal fluid dynamics don't apply since the powder burn rates will screw with any STATIC estimates, and keep in mind the military had fits trying to get different weight/diameter bullets to produce 'Optimum' gas pressure pulse & duration to properly cycle in the different barrel lengths.
 
…or rather, fluid dynamics DO have to be applied, which is why static calculations fall short. The pressure never equalizes fully on the opposite sides of the port opening before the bullet clears the muzzle and bore pressures start to drop. In the extreme, measured pressures in the Garand gas cylinder peak at around 1100 psi, even though the bore sees more like 8000 psi at the 22.5" position in the barrel.
 
Fluid dynamics models will need a specific pressure, which you WON'T have without direct sampling data from all powders, which to my knowledge doesn't exist.

*Some* (very few) powder manufacturers *Might* have data on pressure/burn rates on powders specifically aimed at AR style rifles, but I'm pretty sure they wouldn't include PORT pressures.
Never seen a single powder manufacturer or ammo manufacturer produce PORT pressure estimations.

Without basic input data, no program can estimate PORT pressure.
 
Unclenick:

You might want to change that to 72 F in 3.8 Port Pressure as opposed to 720F.

We, typo part of the original from the looks?
 
You must log in or register to reply here.
Back
Top