Definitions of short and long stroke gas op

[rugerdude]

Okay, so I recently encountered a definition of these two systems from a reputable source that is quite different from what I've always believed to be true. A google search turned up a bunch of wiki stuff that I'd rather avoid right now.

My understanding is that short stroke gas operation involves a piston that is unattached to the bolt/bolt carrier and the actual movement of the piston being very short made it short stroke (AR-18, SVD, SKS, ACR, G36).

Long stroke as I understand is a system involving the piston being directly attached to the bolt/bolt carrier and the long movement of the piston recoiling fully with the bolt carrier made it long stroke (AK, M240, SAW)

The definition that I just discovered is that short stroke is defined by a rifle taking gas from the middle of the barrel and is called short stroke due to the short time that gas is actuating the piston before being bled or cut off regardless of the configuration of the piston, thus making the AK and AR-18 both short stroke.

The definition for long stroke given was a system that takes gas from near the end of the barrel to actuate a piston and that the longer period of gas actuating the piston makes it long stroke.

Can anyone quote a book or something else reputable to settle this? One problem I have with the new definition is this: The AK-47 system fits the new (to me) definition of short stroke gas operation what with the gas port being sorta at the middle of the barrel, but its little brother the AKSU-74 (better yet, pistolized AK variants such as the romanian draco), having a shortened barrel would fit the definition of long stroke despite the whole system being virtually the same.

Can anyone point me in the direction of truth citing a reliable source?

 

[ChronoCube]

What's the source that you mention? It would help if you showed us a link or a verbatim quote.

In the definition of long stroke, which end of the barrel are you talking about? My understanding of what you wrote, based on the part about short-barreled AKs, is that the closer to the muzzle the gas port, the longer-stroke the gas system is, and more time is taken when the gas is pushing the piston. This is not mechanically correct because a gas port near the muzzle would result in nearly zero stroke time.

 

[Ian]

Short stroke means the piston moves less than the length of the cartridge (like the VZ58), and long stroke means that it moves more than the length of the cartridge (like the AK). A short stroke piston is not affixed to the bolt, so that the bolt can continue to travel rearwards after the piston has finished moving.

For the same reason, Maxim and Browning machine guns are called short recoil operated - the barrel assembly moves under recoil, but less than the length of a cartridge.

 

[rugerdude]

The reference that cited the apparently false definitions was a collection of USMC Foreign Weapons Instructor Course powerpoint slideshows used in their classes.

The definition was this: "A short-stroke gas system is defined as one which uses high pressure gas from the middle portion of the barrel that impinges on the piston head for a short period of time before excess gas is either cut-off (M14) or vented (AK-47) or the piston head reaches a stop (M1 Carbine). The distance the piston travels under pressure is generally less than its diameter The piston may or may not be attached to the bolt carrier. This is the most common type of gas operation."

"A long-stroke gas system is generally defined as one which the stroke of the piston under pressure is greater than its diameter. Because of the greater dwell time, gas must be ported from the barrel very near the muzzle of the weapon as in the M1 Garand. This relatively lower pressure gas acts over a longer period of time to impart the same amount of energy to the operating system. Because the operating parts are longer, they are necessarily heavier and this system is not used in modern weapons."

The other information was all apparently correct, so it was a bit like reading that the earth isn't round in a national geographic. So incorrect and yet so believable.

Any reliable source to say otherwise? I would go through my own gun encyclopedias but unfortunately I'm in Afghanistan.

 

[Vern Humphrey]

The short stroke piston was invented by David "Carbine" Williams. The term "long stroke" is a retronym (thought up after the fact.)

The short stroke piston strikes a counterweight, transferring it's momenum to the weight, rather than driving the action itself.

 

[briansmithwins]

Most everyone I've read uses analogous terminology for both gas operated and recoil operated systems: If the moving part (gas piston or barrel) moves longer than the length of the cartridge case it's long stroke. Shorter (than 1 case length) is short stroke.

Attachment or not of the piston to the op rod doesn't figure into it.

BSW

 

[FlyinBryan]

If the moving part (gas piston or barrel) moves longer than the length of the cartridge case it's long stroke. Shorter (than 1 case length) is short stroke.

theres an actuating system that moves shorter that the case?

 

[briansmithwins]

theres an actuating system that moves shorter that the case?

Yep, lots of them:

M1 Carbine
Nearly every tilting barrel semi-auto pistol
MG42

and the granddaddy of them all- The Maxim MG

BSW

 

[Z-Michigan]

Short stroke means the piston moves less than the length of the cartridge (like the VZ58), and long stroke means that it moves more than the length of the cartridge (like the AK). A short stroke piston is not affixed to the bolt, so that the bolt can continue to travel rearwards after the piston has finished moving.

This is the definition I've always heard and read.

 

[Vern Humphrey]

theres an actuating system that moves shorter that the case?

Yes, the short-stroke piston moves only a fraction of an inch and strikes a counterweight, which then transfers the momemtum to the action.

"Long stroke piston" is a retronym -- invented after the fact.

A long recoil system keeps barrel and breech block locked together all the way back, then the barrel unlocks and moves forward. A short recoil system has barrel and breech move back only far enough to unlock the breech block, at which point the barrel stops and the breech block continues rearward.

At the moment, I can't think of any modern long recoil systems -- they were primarily used for shotguns in the early 1900s to soak up the recoil momentum to prevent parts breakage.

 

[FlyinBryan]

lol, if only i had bought that m1 carbine when i was at the cmp i would have known that