Explanation of Short Stroke Gas Piston

[hso]

 

[Tommygunn]

Interesting ... didn't realize that the ss system could be that complex, or be used in a handgun.
My main familiarity with this system is in my inherited M-1 carbine. That system is particularly simple.

Thanks for posting this!

 

[REB]

The Desert Eagle uses a similar piston design.

 

[CapnMac]

That was a good video. I half expected to see some other short-stroke piston examples, like the M-1 Carbine, but, the video was probably better for being to the point.

 

[roscoe]

So what are the advantages of short-stroke design over the long-stroke? It occurs to me that, since the piston is not sitting above the chamber, one advantage would be that you could load from the top using stripper clips.

 

[Deus Machina]

Short-stroke pistons don't need as much space or weight, for one. They certainly don't attach all the weight they do have to the bolt carrier, which can cut down on moving mass and therefore felt recoil.
They can be more finely tuned; they only need to move the bolt, carrier, and spring the full distance, and don't need to move the mass of a piston that needs to be stronger to handle it all that distance, too.
Also along those lines, in their simplest designs they don't need to carry enough energy or mass back, which can slam the carrier back harder than the short-stroke designs (my Saiga AK used to make a nice clank meeting its rear trunnion) so they can last longer and/or require less material to prevent that wear.

 

[lysanderxiii]

So what are the advantages of short-stroke design over the long-stroke? It occurs to me that, since the piston is not sitting above the chamber, one advantage would be that you could load from the top using stripper clips.

Whether the piston is above or below the chamber is immaterial to whether the system is a long or short stroke piston or whether you can feed from the top or bottom.

The M1, for example, is a long stoke piston placed under the barrel that feeds from the bottom, but also allows for top loading.

The major advantage is the reduced mass of the reciprocating parts. Their disadvantage is that they have to have a (relatively) high initial thrust force to get all to parts moving so as to have enough momentum to work the action, whereas the long stroke design can be designed to have a very low, but continuous, push.

Gas-operated, belt-fed, machine guns usually have the long stoke design for this reason. They need a longer 'power-stroke' to lift the belt weight.

 

[roscoe]

Whether the piston is above or below the chamber is immaterial to whether the system is a long or short stroke piston or whether you can feed from the top or bottom.

The M1, for example, is a long stoke piston placed under the barrel that feeds from the bottom, but also allows for top loading.

The major advantage is the reduced mass of the reciprocating parts. Their disadvantage is that they have to have a (relatively) high initial thrust force to get all to parts moving so as to have enough momentum to work the action, whereas the long stroke design can be designed to have a very low, but continuous, push.

Gas-operated, belt-fed, machine guns usually have the long stoke design for this reason. They need a longer 'power-stroke' to lift the belt weight.

So is there any functional difference, say, in felt recoil? Or in the acceptable range of cartridge loadings?

 

[lysanderxiii]

So is there any functional difference, say, in felt recoil? Or in the acceptable range of cartridge loadings?

As to felt recoil, that will always be dependent on the specific design. You can make a short stroke gas piston with a nasty, sharp, uncomfortable recoil or a soft, easy recoil. It all depends on the design....

All gas operated weapons will only work best within a certain range of port pressures, and that is generally governed by the choice of propellant.

 

[CapnMac]

The M1, for example, is a long stoke piston

Well, it's not, even though it resembles one. Because the gas port is near the muzzle, the gass pressure on the piston head only lasts until the projectile clears the muzzle, then the gas pressure drops.

The Piston, since it's attached to the op rod does travel the full length of a cartridge, but that was mostly because Garand did not want to introduce a second part separation.

 

[lysanderxiii]

Well, it's not, even though it resembles one. Because the gas port is near the muzzle, the gass pressure on the piston head only lasts until the projectile clears the muzzle, then the gas pressure drops.

The Piston, since it's attached to the op rod does travel the full length of a cartridge, but that was mostly because Garand did not want to introduce a second part separation.

The definition of a "long stoke piston" is that the piston moves the length of the action, NOT how long the pressure pulse lasts... The Bang rifle had a gas trap muzzle device and a long stroke piston, the bullet had left the barrel by the time gas started acting on the piston. (Besides, the pressure in the barrel remain high, above 4000 psi, for a few milliseconds after the bullet has left the barrel. Complete blow-down takes about two times the length of time the bullet is in the barrel.)

The definition of a "short stroke piston" is that the piston move a fraction of the action length.

Long stroke pistons:

M1
AK designs
MAG 58
FG 42
M1918
Hotchkiss MG

You will find that all of the above designs vent all gas from the cylinder after about .25 to .75 inch of piston travel.

[If you look inside the M1 gas cylinder, you will see that the inside diameter increases after about the first 1/2 inch. This allows gas to vent around the piston head.]

 

[lysanderxiii]

This is the pressure-time curve for 7.62mm NATO out of a 20 inch barrel. You will note the pressure is still about 10,000 psi at bullet exit and it takes about 1.3 milliseconds eject the bullet, but the pressure doesn't drop below 4,000 psi until another 1.3 milliseconds.

 

[zaitcev]

Their disadvantage is that they have to have a (relatively) high initial thrust force to get all to parts moving so as to have enough momentum to work the action, whereas the long stroke design can be designed to have a very low, but continuous, push.

The advantage of having the longer duration of the power delivery in the gas engine is somewhat academic for rifles, because you can only do it by attaching a large enough vessel in front of the piston. The maintenance nightmares of dealing with it made designers give up on the idea. So, in all modern long- or short-stroke guns, the energy is delivered to the reciprocating mass in a process similar to a blow with a hammer. This even includes belt-fed machine guns.

 

[LRDGCO]

SO. NOT. COOL.

Have had my little handgun acquisition...errr...problem under control for a while now.

And then you show me this utterly BA, and entirely impractical Battlestar Gallactica awesomeness: the Wildey Survivor pistol (even the name is appealingly absurd).

Now I have to have one. Bad hso!

 

[lysanderxiii]

The advantage of having the longer duration of the power delivery in the gas engine is somewhat academic for rifles, because you can only do it by attaching a large enough vessel in front of the piston. The maintenance nightmares of dealing with it made designers give up on the idea. So, in all modern long- or short-stroke guns, the energy is delivered to the reciprocating mass in a process similar to a blow with a hammer. This even includes belt-fed machine guns.

Not a hammer blow. If you look at the force vs displacement curves for most long stroke MGs they are longer duration than say an M1 Carbine, or even a FAL, several milliseconds. Even most short stroke pistons have a relatively long duration pressure pulse.

The difference of a few milliseconds (or even fractions of a ms) may seem to be nothing, but it changes the accelerations on the moving parts to a great extent. Calculate the required acceleration to get a half pound mass going about 10 feet per second with a pressure pulse of 1.5 ms or 3 ms, it's half, the same goes for distance, double the distance the piston moves under pressure, the acceleration drop proportionally.

For example, look at a baseball, when thrown by a good pitcher the ball can reach speeds of 90 mph, and when hit by the bat (from a Tee-ball stand), it reaches about the same speed. Both accelerations occur within the blink of an eye, but put a watch inside the baseball - in which case would the watch mechanism more likely survive in working condition?