SVD vs. AK Piston Systems and Accuracy
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Thread: SVD vs. AK Piston Systems and Accuracy

  1. #1
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    Default SVD vs. AK Piston Systems and Accuracy

    I was asked by a member to comment on the superior accuracy of the SVD piston system, specifically if the piston actually moves before the bullet leaves the barrel. I thought the more technical members might enjoy this as it directly shows a major difference between AK/PSL-based rifles and SVDs (and the Wiki moving drawings are pretty cool.).

    I said, yes, the piston must take the rearward energy required to set it in motion from its previously inert position and it begins to move as soon as the pressure from the gas from the barrel hole hits it, actually before the bullet has left that long barrel, in microseconds. (Compression of previous gas or air in the forward tube impacts the piston even faster than the hot new gas behind it.)

    The heavier long stroke piston's rearward motion (and impact of gas required to set it in motion) of a Kalashnikov design is more disruptive to accuracy (technically the term is "precision," but who cares) than the much lighter SVD piston system. Receiver and barrel flex aggravate this.

    Amazing high speed photography shows all this happening as rifles whip around like spaghetti as recoil begins very early in the cycle. It seems to the shooter that nothing happens until it goes bang, but the rifle has already taken quite a whack to set the bullet in motion in the first place.

    A primary accuracy difference between the PSL or AK system and the Dragunov is the piston design, a short stroke SVD versus a heavier long-stroke PSL and AK.
    Obviously it takes a lot less energy going backwards to move the light piston and, of course, that energy really doesn't go in a straight perfect line. It actually goes in all directions, though the piston tube confines it to rearward, the energy goes every which way as the tube directs the piston rearwards for a substantial move from a dead stop. Though the piston seems tight in the tube it actually bounces around like a pinball in there and flexes itself.

    The book Tactical Small Arms of the 21st Century, by Charles Q. Cutsaw explains how the SVD piston works on p. 276:

    "The assault rifle and other Kalashnikov small arms use a long stroke piston that is generally unsuitable for a precision rifle since the movement of the fairly heavy mass mitigates against extreme accuracy. Therefore, Dragunov used a short stroke piston system. The lightweight piston is driven back by the gas impulse and transfers energy to the bolt carrier, moving it to the rear. A lug on the bolt carrier engages a cam path on the carrier, rotating the bolt to unlock it. The carrier and the bolt travel back together....."

    In other words, the energy transfer and movement of the piston system must be complete before the bolt can open. As this energy transfer starts before the bullet leaves the barrel, all sorts of flexes and motion occur in the entire rifle, with a heavier piston (and a more flexible receiver) in the AK platform or PSL getting worse motion than a much lighter piston coupled with a very stout receiver in a real SVD, NDM-86 or TIGR.

    Needless to say, a Stoner-based "gasser" rifle uses direct gas impingement and no piston at all, a theoretically more precise system than either of the above.

    Here is Wikipedia on the piston topic.
    \
    Long-stroke piston


    Diagram of long-stroke gas operation system



    Long stroke gas piston, from an AK-74.


    With a long-stroke system, the piston is mechanically fixed to the bolt group and moves through the entire operating cycle. This system is used in weapons such as the Bren light machine gun, AK-47, Tavor, FN Minimi, M249 Squad Automatic Weapon, FN MAG, FN FNC, and M1 Garand. The primary advantage of the long-stroke system is that the mass of the piston rod adds to the momentum of the bolt carrier enabling more positive extraction, ejection, chambering, and locking. The primary disadvantage to this system is the disruption of the point of aim due to several factors such as: the center of mass changing during the action cycle, abrupt stops at the beginning and end of bolt carrier travel, and the use of the barrel as a fulcrum to drive the bolt back. Also, due to the greater mass of moving parts, more gas is required to operate the system that, in turn, requires more massive operating parts.

    Short-stroke piston


    short-stroke gas piston



    Short stroke gas piston and bolt carrier group, from a gas piston AR-15.


    With a short-stroke or tappet system, the piston moves separately from the bolt group. It may directly push[2] the bolt group parts as in the M1 carbine or operate through a connecting rod or assembly as in the Armalite AR-18 or the SKS. In either case, the energy is imparted in a short, abrupt push and the motion of the gas piston is then arrested allowing the bolt carrier assembly to continue through the operating cycle through kinetic energy. This has the advantage of reducing the total mass of recoiling parts compared to the long-stroke piston. This, in turn, enables better control of the weapon due to less mass needing to be stopped at either end of the bolt carrier travel.
    Last edited by Stalin's Ghost; 04-24-2017 at 03:13 PM.

  2. #2
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