How much energy does the gas system soak up?

fusiachi

PFC Snuffy
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May 12, 2008
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Since pressure peaks well before the bullet passes the gas port in my rifle, is the energy bled off by the gas system likely to have an appreciable effect on internal ballistic performance? I was in the process of adjusting QuickLoad to reflect lower than expected chrono results for a .223 load when it occurred to me that QL might be predicting the pressure level accurately and the velocity shortfall might be found in the gun's operating system.
 

Hummer

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Jun 20, 2009
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Re: How much energy does the gas system soak up?

Fusiachi
You are correct the highest peak pressure occurs with the bullet out of the case mouth. Depending on propellant, the distance varies.
Gas guns need a certain amount of pressure to drive the system. Overdriving the system is bad as it will tend to wreck your rifle quickly.
The nice thing about Quick Load is you can check the propellant you are using to make sure you are within the window.
The problem is knowing how much pressure is enough and how much is too much.

For M1A max port pressure average should not exceed 11,800 PSI.

For M1 Garand the pressures run from 8000 to 11,000 PSI.

I had to use QL to figure port pressure for my Tanker Garand. As soon as I shot it I realized the factory loads were way too hot for a tanker.
The short barrel obviously has the gas port closer to chamber thus higher port pressure was encountered. I wanted my loads in the lower 50% of the range and I was able to find a load that gave 7000 lbs (per QL) and still work the action. Then again if you were going to be in the mud, blood and beer party you want more "action" to make sure the rifle gets enough gas to work in all conditions from sub zero, mud, dust etc.

It was going to be a touchy deal so I fell back on plan B which was to make a vented gas port for the Garand and kept opening the hole until I got ejection between 1:00 and 2:00 with milspec ammo.
I can now shoot 150 and 173 milspec ammo with same ejection pattern.

The MTU did similar tricks with M14 Match rifles by drilling the gas plug to vent off higher pressures generated by their long range loads and they also cut slits down side of gas piston to allow gas leakage forward.

The AR family has a higher pressure requirement of around 16,000 lbs. Care must be taken here so as not to get a shorty barrel with same size port as standard length barrel. The first run of M4s had same size gas ports and literally wrecked brass. All the case rims were deformed due to the violent extraction forces the cases saw. The later contracts had reduced port size and the problem went away. I ran into this from a put together rifle and after market barrel (make unknown) that had the big port and it wrecked a bunch of case rims.

While QL doesn't have a port pressure heading as such it can be figured by using the distance to the port as the barrel length and read the muzzle pressure as your port pressure reading. Least ways it works for everything I have run.

Be advised on M1 Garand loading surplus propellant you may run into WC852. There are slow lots and normal lots. The slow lots gave excessive port pressure in Garand loadings so this propellant was restricted to the Browning MG ammo runs only. Thusly if you run up on belted 30.06, it may not be suitable for a Garand by pulling belt down.
By the way WC852 was excellent for 6.5X284 and magnums as it gave high velocity with low pressure.

As a side note military ammo is loaded to a pressure/velocity standard. If pressure is exceeded before velocity is met it is rejected. If velocity is met and pressure is too low it is rejected. On the WC852 Slow lots pressure was only 44,000 lbs on chamber but way high at port thusly the restriction to BMG load runs.
Unfortunately due to the variation in barrels you can't be sure what you are seeing. For instance you might take the same load and shoot in M1A and a bolt gun with same barrel lengths. The M1A can show higher velocity with same ammo due to variance in bore dimensions.
There is no controlling factor on after market barrel manufacturers thusly the land and groove dimensions vary. This is easily discerned with a good bore scope. Some barrels show maybe 50% contact of projectile in grooves all the way to muzzle, some part way to muzzle and some maybe only 10% groove contact. Obviously the lesser the groove contact the more gas slips by the bullet giving you less ability to build pressure.
Trying to figure pressure loss is akin to trying to figure out what sorry low life politicians will do next.
 

fusiachi

PFC Snuffy
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Re: How much energy does the gas system soak up?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hummer</div><div class="ubbcode-body">
While QL doesn't have a port pressure heading as such it can be figured by using the distance to the port as the barrel length and read the muzzle pressure as your port pressure reading. Least ways it works for everything I have run.</div></div>

Thanks for that; not sure I would have figured that one out on my own.


<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Trying to figure pressure loss is akin to trying to figure out what sorry low life politicians will do next. </div></div>

Sadly, what they'll do next is becoming easier and easier to predict. Thanks for the detailed answer to my question.