The SVD & short stroke gas system & why

[Ekie]

The Soviets deployed a purpose built sniper rifle in the late 1960s commonly referred to as the SVD. Dragunov’s design won out over other submitted rifles including one by Kalashnikov. The actual details on why Dragunov’s design was chosen over others is a mystery to me. Maybe others can enlighten us. The biggest difference between the Kalashnikov and the Dragunov design is the operating system. The Kalashnikov use a long stroke gas piston system while the Dragunov uses the short stroke piston.

While discussing the merits of both systems via e-mail with another member a fundamental issue involving gas systems came up. I thought it best that this should be discussed in a public forum so that others could join in.

So why did the Soviets go with the short stroke? My theory is that this system eliminates two harmonic issues involving the barrel. With the long stroke set up it is difficult to get the long gas piston/carrier assembly to come to rest at the same point after each cycle. This has some importance in that this large assembly is in contact with the barrel through the gas tube and gas block. This mass will vibrate with the barrel after a shot has been fired. The corresponding harmonic vibration will be different depending on the resting point of the carrier/piston assembly.

The other advantage to the short stoke in terms of barrel harmonics is what has brought up an interesting argument. I maintain that after the bullet has traveled past the gas port that the gas pressure behind the bullet begins to move the gas piston assembly. Others maintain that no such movement occurs until after the bullet has left the barrel. If any parts move in a long stroke piston assembly while the bullet was in the bore there would be a corresponding change in barrel harmonics that would be measurably detrimental to accuracy, due to the mass of a long stroke gas piston assembly. Hence the SVD uses a short stoke piston operating system.

[FN49]

Hey,Ekie; why are you posting this here instead of on another site (that shall remain nameless)? http://www.ak-47.net/akcgi/wink.gif

Anyway, the standard answer to this question is that the greater mass involved in a long-stroke gas operated system changes the rifles center of gravity during the shot and reduces the accuracy compared to a short-stroke system. This assumes,of course,that the piston is moving before the bullet leaves the barrel.

Good topic; I'd like to see more of this kind stuff posted (but not neccessarily here http://www.ak-47.net/akcgi/biggrin.gif ).

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FN49 aka M77B1

[Ekie]

Yeah, I could have started it over there. I have had good luck with starting interesting threads (to me)here in the past. Old habits die hard.

So FN49 looks like you are not going to go out on a limb as to whether the operating systems starts moving before the bullet leaves the muzzle?

[FN49]

If I had to guess,I'd say the piston does start to move before the bullet leaves the barrel,but I don't have any evidence to back this up. I think it might also depend on how close to the muzzle the gas port is located. The piston of an Ak would likely move earlier than the piston on a design like the FN49,which has it's gas port closer to the muzzle than the Ak.

There must be some reference work that has studied this somewhere,don't you think?

[Chamberlinx]

I'm guessing if there is enough pressure
to push a bullet out the bore, there is no
reason that same pressure would'nt find it's way to the gas chamber as well; especially since the barrel pressure is greater when the bullet is still inside. Of cousre, a certain
amount of gas is required to fill gas port
volume before any movement can happen, so plenum size and port placement are factors too. If you ask me, I think the majority of the force impinged on the piston happens BEFORE the bullet leaves the muzzle. After that, only residual pressure is chasing the bullet out; being weaker, and without a plug
in the end of the barrel, I would think this
has little affect to the piston. By the way,
where is this "other" place. I like it here!
Don't tell me it is the one with the black rifles....my old stomping grounds...
-C

[This message has been edited by Chamberlin (edited 08-09-2000).]

[Ekie]

FN49

Glad to hear you take a stance.

Chamberlin

I must confess that I am a fan of the direct gas impingement system and the popular Stoner rifle that uses it, and I do stop in at the other board you are referring to. Fact is little is going on in that community. Here in AK land there is a lot going on and a lot to learn. Times have never been better for info on the Kalashnikov, and the availability of exotic Kalashnikov parts improve every day. FN49 is referring to a new site dedicated to Kalashnikov fans. It has gotten a good start but is still less then two months old. Many names familiar to you are there.

[Ekie]

Oh yes almost forgot the topic. I know of no source for information on this subject or I would site it. Chamberlin the gas port hole in a barrel has little to no effect on bullet velocity. This would suggest that the pressure behind the bullet is not effected by gas being diverted in to gas port.

[Chamberlinx]

No Ekie, there hasn't been much going on over at the other famous assault rifle forum.
I built my 24" SS 15 lbs. match AR, then my
M4 clone, and pretty much called it a day.
There is so much more history surrounding the AK- it's especially fun since I have access to some original Soviet stuff.
With regard to the gasport topic, I just finished restoring a 7mm Venezuelan FN-49 this weekend (speaking of FN-49's!) and I tuned it with the top handgaurd off. This got me thinkning how neat it would be to find someone like the late Dr. Egerton to film high speed movies of the muzzle/gas tube area
during firing to see what really happens, with out speculation. I love watching those old super slow military films of arms in action. With regard to the gas port vs. bullet velocity, I'm still a little unsure.
It seems to me the bullet, having reached the
gas port, has attained most of it's velocity
already (explaining the only "slight" drop in FPS) and from here, I wouldn't make any hard conclusions about the gas behind the bullet not being effected by the gas port. In other words, I wouldn't put the bullet velocity too deep into the equation. Afterall, how could backpressure *NOT* be effected by tapping off some gas to the port? Obviously, the location of the port relative to muzzle would be a factor, as FN has said.
Next piece of food for thought.....Blank Firing Adapters. -C

[This message has been edited by Chamberlin (edited 08-10-2000).]

[CAMPYBOB]

short, sweet explaination (second time for ekie...heheh!). the drag was designed with a short stroke gas system for the purpose of keeping the rifle rifle "balanced" during recoil and autoload cycling for quick re- aquisition of a follow-up shots (or multiple target engagement....thus the semi-auto concept as applied to a sniping rifle). and yes, the bullet is long gone from the bbl. by the time the gas piston starts to move rearward. even after the projectile has left the bbl., the superheated combustion gases continue to expand...actuate the gas system, exit the muzzle and breech (after open...by this time pressures have dropped off, but are still far above atmospheric preesure).

[FN49]

Campybob,I respect your knowledge and opinions,but I have to go with W.H.B.Smith and Joseph E. Smith (U.S.Army Materiel Command) in their classic reference work "The Book of Rifles" (1948;revised 1963).

Using the M1 Garand as an example,they state that after the bullet has passed the gas port the piston/op.rod starts moving rearward BEFORE the bullet leaves the barrel (and the gas port is very near the muzzle on the M1). The op.rod has about 5/16" of free travel which gives the bullet time to clear the muzzle and allows the breech pressure to drop; then the op. rod cams the bolt to unlock it.

[This message has been edited by FN49 (edited 08-10-2000).]

[This message has been edited by FN49 (edited 08-10-2000).]

[CAMPYBOB]

fn49...highly possible. atmospheric gas in the bbl. may be "pre-presurized" by the forced compression of the bullet travelling towards the gas tap hole...and starting the "unloaded" op rod into reverse. the drag's rod is direct coupled to the carrier and is spring loaded (by the action recoil spring) forward. the mass of the bolt and carrier assemblies are high on the drag.

let's assume a bullet velocity of 2300 fps...or 27,600 inches per second. assuming that a drag's gas tap is 12" from the muzzle opening, this calculates out to .00043 of one second that the bullet is still in the bbl. and past the gas tap port. in .00043 (that's 4 ten thousandths of one second) the gas must first filled the vacant xx cubic in. of cavity, presurize and preload the gas system, overcome all three forces, inertia, friction and spring preload...and start the rod/piston travelling rearward. even in the long bbl. drag ... .00043 is not a lot of time for a change in mass about the rifl's polar moment of inertia to affect accuracy.

i feel that short stroking the piston was done to maintain the rifle's balance for follow on shots...after all, that is the only advantage offered by an autoloading sniper. even our garand m1c's and d's, with their conventional op rod setups (and the ultra accurate m1a, for that matter), offer more than adequate first shot accuracy standards for sniping at ranges that equal the svd. it is what happens "after" you get that first shot off that differentiates shoot a drag from an ultra match or m1d.

as a side note: ever shoulder or shoot a drag? notice the "nose heavy" attitude (in offhand, kneeling...not bench) compared to an m1a? despite the "short stroke" setup dragunov came up with. imho, the russians still found it necessary to keep the center of mass forward to control recoil rise. again, i would postulate that this is another purposeful design feature oriented at providing the shooter with a "better" second shot, so that he can take full advantage of his rifle's autoloading feature.

just my 2 cents...your milage may vary!

[FN49]

Campybob: I agree that any movement of the piston before the bullet leaves the barrel probably does not cause enough change in balance to effect the shot,although it can't be ruled out entirely. I also agree that the short-stroke system's main advantage over the long-stroke system is that it allows for better follow-up shots. I was just introducing an expert's statement on the point of whether or not the gas behind the bullet starts the piston moving before it leaves the barrel.

BTW- I've revised the statement by the two Smith's in my post above; I failed to include that the piston/op.rod begins movement after the bullet has passed the gas port (but before it leaves the barrel).

Good analysis,Campybob.Enjoyed it.



[This message has been edited by FN49 (edited 08-10-2000).]

[CAMPYBOB]

fn49...this is an iteresting topic (and ekie felt more guys could enjoy it and comment on it than just his and my e-mails). i am, by no stretch of the imagination, a firearms expert (disclaimer?)...but i do remember reading an article wherein gas systems where disabled in several types of rifles and no increase in first shot accuracy was achieved.

as an "ex" high power shooter (familiar with most of the bench "voodoo" [ekie's term...and i like it!) of semi's and bolt guns, i always appreciate a good "theoretics" debate. i have always maintained that the greatest accuracy benefit achieved by a bolt over an autoloader (all things being equal...and, by golly, if you blow enough cash on either system they can be!) is in the dimensinal control and repeatability of the chamber that the bolt gun provides. the semi, by nature of the beast, always varies a few thousandths.

[Ekie]

Oooohhh good thread. I will not be able to respond until later tonight. I am just stopping in over lunch.

CAMPYBOB you sighted tests that show no differance in accuarcy with the gas system disabled. I must ask, what gas systems did these rifles use?

[FN49]

Good point about bolt vs. semi headspace.

You da' man,Campybob.

[krinkovx]

well i still like my svd.and with the b-15 soviet sniper ammo it still will shoot fine for my satisfaction,and i never protested to be a 1,000 yard match shooter,just a well seasoned shooter who will take the shot if need be in a bad situation,not at a paper target but one that will shoot back,and to me that is the bottomline,take care,krinkov

[CAMPYBOB]

krinkov...don't shoot!...don't shoot! we're on your side!!!

[Ekie]

Gas Operation

I got a chance to review various gas operating systems as explained in multiple publications tonight. There is no controversy on whether any part of the operating system moves prior to the bullet exiting the muzzle. There is a gas system that does not move until the bullet leaves the barrel, but it is of little consequence.

The long-stroke gas system is one of the most popular for post war small arms. It was made famous by the M1 Garand. It is also used in the Kalashnikov. It works as follows. Rapidly expanding gases behind the bullet flows into the gas port after the bullet has passed the gas port. The pressure behind the bullet is what works the action. After the bullet leaves the muzzle pressure drops to that of ambient atmospheric pressure rapidly. The expanding gases push on the front of the gas piston and shoves the bolt carrier back against its action spring. At this point where the expanding gas has shoved the carrier back and before the bullet exits is the most critical and where the majority of the force that works the system acts. Very little pressure is in the “gas chamber” after the bullet exits. It is vented through the muzzle and vent hole in the gas block (AKM and AK-74). The bolt carrier unlocks the bolt through a camming surface after the bullet exits and pressure reashes a safe level. This is why there is a delay in unlocking the breach.

With the long-stroke system as long as the gas port is near the face of the gas piston, gas port location is not as critical as it is in other gas systems. The M1 Garand, AKSU, and the Micro Galil all have the gas port located near the muzzle, and are reliable in operation. The Stoner dirative of the direct gas impingement system (an Eklund design I might add) as used in the AR-10/AR-15, is much more sensitive to gas port location. This is why the XM177 was dropped and the M4 Carbine program was initiated. The bullet needs to travel at least four inches past the gas port in order for there to be enough time for the bolt carrier to gain enough momentum to successfully complete the action cycle in a fully automatic mode.

Chamberlin

The effect of muzzle velocity and the gas port. There was an experiment conducted and detailed either in BLACK MAGIC or The COMPETATIVE AR-15. I do not remember which. A custom match AR-15 was shot before and after the drilling of the gas port. There was no difference in projectile velocity. I will admit that the closer the gas porthole is to the breach the more likely there would be a loss in muzzle velocity. In short the gas port is inconsequential in terms of muzzle velocity.

Gas Operation and Barrel Harmonics according to Ekie Voodoo type reasoning

There are three reasons that a bolt gun has an advantage over the gas gun in terms of accuracy. First off the chamber dimension can be held to closer tolerances in a bolt gun, while having little effect on the reliability of the action. Secondly it is more difficult to achieve a consistent lock up of the action of a gas operated rifle due to its inherent complexity. This is important not only in the positioning of the case in the chamber but also in where parts rest in contact with the barrel. The long-stroke operating system is the worst offender. Unlike a bolt action or a direct gas impingement (the retarded gas blow back and recoil operation are exception are a different breed) operated rifle most other systems have moving parts that contact the barrel. These parts will not come to rest in the exact same place after each cycle of the action. This is important because anything touching the barrel will effect barrel harmonics. The longer and more ridged these objects are the more harmonics will be effected. Thirdly there are parts moving while the bullet is in the bore. The bigger and more ridged the mass the bigger the effect will be on harmonics.

The SVD and Ekie’s explanation of its short-stroke gas system

CAMPYBOB your viewpoint that the short-stroke system was chosen because of its inherent advantage in shot recovery is widely shared. I consider it to be of secondary importance, and a side benefit. It seems to me that the system was adopted purely because it makes for a more accurate rifle.

Krinkov

I am pleased to hear that you find your SVD to be satisfactory in accuracy

[krinkovx]

yes it is accurate,but remember it should only be compared to other semi sniper models and not super duper upscaled bolt action rifles,the dragunov is a tough soldier proof rifle,designed with ease of use and reliability in the field in mind,and the chance for a quick follow up second or third shot,like maybe taking out the enemies sniper team.or m.g. nest.the info was very good,take care,krinkov p.s. and campybob you funny guy!!!

[This message has been edited by krinkov (edited 08-11-2000).]

[CAMPYBOB]

ekie, here's a thought...as an aside: if, as you believe was the designer's intent, that the short stroke gas systems purpose was primarily accuracy...why then aren't the handguards floated? common technology in the '70's and of no detriment...or any major cost increase (perhaps even a decrease...ever checked out the handiwork required to make a svd's guardset?) to the rifle. this would, in all probability, contributed greatly to accuracy. and that pencil bbl.? it may be light...but stabile? the svd was a compromise package...as are all military firearms. perhaps the short stroke gas piston really does have an affect on one shot accuracy...but i think the numbers speak for themselves.

[Ekie]

As you say the SVD, as all rifles is a compromise. The important features of the rifle help explain what the Soviets where looking for. The trigger mechanism is based on the Kalashnikov, which is in turn based on the Garand. A simple and proven set up. Why deviate? The receiver is milled due to the additional strength this provides over the stamped receiver. This provides a more stable and accurate platform. . The barrel is thin in order to reduce weight. The short-stroke gas system was chosen over the long-stroke for accuracy potential, faster recovery time, and reduced weight. The short-stroke simply affords the rifle with better accuracy. It also provides a weight saving that allows the rifle to use a milled receiver, again helping accuracy. There is also the benefit of faster recovery time. My point is that there are a number of reasons the SVD is short-stroke operated, and that accuracy of the rifle is the most important attribute.

Yes there are many other features that where left off the rifle that would have made it more accurate, such as free floating the barrel. This feature along with others included in the design would have brought the rifles weight over 10 pounds. This makes it quite clear that lightweight was also a high priority in the desired characteristics of the rifle.