Everything you know is wrong: Impingement vs Gas Expansion

[SteveL]

Interesting. I've read before that the "DI" system as we know it in the AR isn't a true direct impingement system, but I've never looked into the technical reasons why.

[prepare]

Eventually some company will reinvent the wheel and release the latest generation AR with the all new "Gas Expansion System"

[lysander]

I always thought Stoner's original design for the AR10 (which is referenced above) would have been superior to the later "gas key on top of the carrier". It would be interesting if it would have been retained.

Had it been retained, it would not be a reliable system.

The original "Hollywood" AR-10s had the side mounted gas tube that worked almost exactly like the patent drawing. They didn't work as well as advertised. The reason is: there is a conflict between the clearance required for a good gas seal (tight) and the clearance required for minimum friction between the carrier and upper (loose).

If you made the carrier fit close enough to get a good gas seal, after a few rounds the fouling jammed up the carrier, and if you loosened up the tolerance so it could tolerate the fouling, it leaked.

The introduction of the carrier key allowed these two tolerances to be made independent of each other. The key and the gas tube can have 0.0005 to 0.001" clearance, and the upper and carrier can have a 0.0065 to 0.0125 clearance. It also allowed the contact area between the carrier and upper to be reduced, so fouling is even less an issue.

[bobcatdriver]

Had it been retained, it would not be a reliable system.

The original "Hollywood" AR-10s had the side mounted gas tube that worked almost exactly like the patent drawing. They didn't work as well as advertised. The reason is: there is a conflict between the clearance required for a good gas seal (tight) and the clearance required for minimum friction between the carrier and upper (loose).

If you made the carrier fit close enough to get a good gas seal, after a few rounds the fouling jammed up the carrier, and if you loosened up the tolerance so it could tolerate the fouling, it leaked.

The introduction of the carrier key allowed these two tolerances to be made independent of each other. The key and the gas tube can have 0.0005 to 0.001" clearance, and the upper and carrier can have a 0.0065 to 0.0125 clearance. It also allowed the contact area between the carrier and upper to be reduced, so fouling is even less an issue.

Is there documentation in regards to this? I'd be interested in reading about it.

[lysander]

If you can find a copy:

"Evaluation Test of Armalite 7.62mm, Rifle, AR-10," Springfield Armory, 1957-02-04

If you every get ahold of a copy, let me know. All I have is what other people have said about it.

And:

"A Test o Rifle, Caliber 7.62mm, AR-10," Springfield Armory, 1960-11-01

You can see the problem in the photos:

Rifle, 7.62mm Armalite AR-10 - Carbon Fouling Condition of Bolt Group.jpg

The slot on the front side is where the gas tube mates. Also, with the gas key, almost all the gas is directed into the carrier cavity, whereas with the side mount, as soon as the carrier moves back slightly gas starts to blow all over the OUTSIDE of the bolt carrier.

[BufordTJustice]

If you can find a copy:

"Evaluation Test of Armalite 7.62mm, Rifle, AR-10," Springfield Armory, 1957-02-04

If you every get ahold of a copy, let me know. All I have is what other people have said about it.

And:

"A Test o Rifle, Caliber 7.62mm, AR-10," Springfield Armory, 1960-11-01

You can see the problem in the photos:

Rifle, 7.62mm Armalite AR-10 - Carbon Fouling Condition of Bolt Group.jpg

The slot on the front side is where the gas tube mates. Also, with the gas key, almost all the gas is directed into the carrier cavity, whereas with the side mount, as soon as the carrier moves back slightly gas starts to blow all over the OUTSIDE of the bolt carrier.

The slab boss on the top of the carrier is also apparent to me. Thank you for sharing the image. I've only seen very low quality versions of that.

So the gas key as we now know it serves to align/center the carrier within the upper in addition to the gas related functions? Cool.


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[Ned Christiansen]

I had a question posed recently. I felt I knew the answer but just for fun I tested it. Now some may think it a dumb question but not everyone understands how these things work, in fact no one knows.... no one has any idea how anything works... until they find out.

"Would an AR work like a MAS 49/56 or AG42 if the only "drive" was the gas coming out the end of the gas tube into the carrier key? In other words, if the gas had no channel to get any further?"

Now this answer does not exactly, um, impinge on the above discussion but is somewhat related and i thought the test was kinda fun.

I removed a gas key and put a .001" shim under it to block the gas path to the inside of the carrier.

The gun cycled normally, I couldn't believe it! When I took it back apart, I found that, aha, the gas had blown through the .001 steel shim. I tested again using .005 shim. Yes, there could be a slight tube-to-key alignment is but pretty much only theoretically. Certainly there was not enough to notice.

The gun did not cycle, BUT.... it tried. To my surprise the carrier moved back about 1 1/4" the first shot and 3/4" the second shot (both with BHA MK262 Mod 1).

Now ain't that interesting? I pointed out in the answer that although this was indeed rather like the AG42 and the MAS, they both have much more area for the gas to act upon due to a larger diameters of the "pistons". Think for a moment about hydraulic cylinders: a larger cylinder gives more thrust. We can lift a 5,000-pound car with only ~50 PSI if the cylinder is large enough (about 12”). If we use a cylinder whose piston area is 1 square inch, we’ll need >5,000 PSI.

This might be a good method for (roughly) determining the pressure a the back of the gas tube. Lysander, that sounds like your bailywick :-).... in case you'd like to calculate how many PSI it would take to perf .001 steel at the diameter of the carrier's gas entry hole (plus chamfer I reckon). Not presuming to give you an assignment, this is more like.... more like.... bait :-) The steel shim stock is, I believe the 1095 at Rc48 variety.

[kirkland]

If anyone actually made a "direct impingement" system, it would look something like this, and be horribly inefficient.

That would take a lot of gas for sure.

[lysander]

I had a question posed recently. I felt I knew the answer but just for fun I tested it. Now some may think it a dumb question but not everyone understands how these things work, in fact no one knows.... no one has any idea how anything works... until they find out.

"Would an AR work like a MAS 49/56 or AG42 if the only "drive" was the gas coming out the end of the gas tube into the carrier key? In other words, if the gas had no channel to get any further?"

Now this answer does not exactly, um, impinge on the above discussion but is somewhat related and i thought the test was kinda fun.

I removed a gas key and put a .001" shim under it to block the gas path to the inside of the carrier.

The gun cycled normally, I couldn't believe it! When I took it back apart, I found that, aha, the gas had blown through the .001 steel shim. I tested again using .005 shim. Yes, there could be a slight tube-to-key alignment is but pretty much only theoretically. Certainly there was not enough to notice.

The gun did not cycle, BUT.... it tried. To my surprise the carrier moved back about 1 1/4" the first shot and 3/4" the second shot (both with BHA MK262 Mod 1).

Now ain't that interesting? I pointed out in the answer that although this was indeed rather like the AG42 and the MAS, they both have much more area for the gas to act upon due to a larger diameters of the "pistons". Think for a moment about hydraulic cylinders: a larger cylinder gives more thrust. We can lift a 5,000-pound car with only ~50 PSI if the cylinder is large enough (about 12”). If we use a cylinder whose piston area is 1 square inch, we’ll need >5,000 PSI.

This might be a good method for (roughly) determining the pressure a the back of the gas tube. Lysander, that sounds like your bailywick :-).... in case you'd like to calculate how many PSI it would take to perf .001 steel at the diameter of the carrier's gas entry hole (plus chamfer I reckon). Not presuming to give you an assignment, this is more like.... more like.... bait :-) The steel shim stock is, I believe the 1095 at Rc48 variety.

The gas ports on the MAS and Ljungman are pretty big compared to traditional piston designs.

The peak pressure at the gas tube/key interface depends, of course on the gas tube length, port diameter, post-port barrel length, reciprocating mass, spring rate, and the pressure-time curve for the ammunition type.

For a 20 inch barrel with a USGI rifle gas system, shooting 5.56mm, M855 ball, with a standard USGI rifle buffer and spring: around 2,800 psi

For a 14.5 inch barrel with a USGI carbine gas system, shooting 5.56mm, M855 ball, with an H2 buffer and standard spring: around 2,600 psi

The normal force pushing the carrier back is around 400 to 500 pounds

With just the area of the gas tube to work with, you were getting about 70 to 80 pounds of force.