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View Full Version : Technical: direct gas operation a la AR?


Dano
August 07, 2000, 12:05
Anyone ever think to remove the piston and replace it with a simple gas tube (like an AR) that protrudes 1/4" or so past the receiver face. Drill a corresponding hole in the front of the carrier to receive the tube?

Would there be enough gas pressure to cycle the bolt carrier? Maybe modify barrel port to increase gas flow to make it happen?

Why do all this, you say? Because the mass of piston cycling prevents the FAL from being as accurate as it can be.

If this works, the market would be the high-profit target shooters. Think it is possible? I would be interested in being involved on this R&D, if you think it has even a remote chance of it working.

Yes, I would be willing to supply $$ and parts. It's time to refine the FAL and open its markets. It's a great rifle.

[ August 02, 2001: Message edited by: gary.jeter ]

[ August 07, 2001: Message edited by: EMDII ]

recce
August 07, 2000, 13:19
I'm sure a competent smith or machinist could do it but then that would cause fouling problems in the receiver which would be worse than the M-16 due to the gas being vented into the bolt carrier only, unless you drilled some vent holes in the top or side of the carrier. In which case you would have gas venting uncomfortably close to the shooters eye. I'm not a smith or anything, but that's what I see happening. Besides, why mess with near perfection?

derek huffman, azexarms
August 07, 2000, 13:32
I think the assumed accuracy would be ruined by 1. stresses on the gas sytem far in excess of what they are designed to handle, the mass of the piston "tames" the flow of gas somewhat. and 2. The accompanying dirtyness of the action would render it as finicky as some ARs. If it ain't broke, dont fix it. D.

Dano
August 07, 2000, 14:17
Accuracy ruined? Come on now. A FAL is at its most accurate when the gas is off.

Don't believe me, go try it. Every FAL I have (and I have several), shoot much tighter groups with the gas off.

The fouling problem isn't much of a concern to target shooters. They clean frequently.

As far as the gas venting close to the shooter's face, it wouldn't be much different than an AR. I don't think anyone thinks that they're dangerous.

They would also probably pay a premium for the modification.

Come on Derek, where your sense of adventure and challenge? This could be fun.

I can see the shooting line at Camp Perry now -- filled with FAL's taking home the trophies.

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

recce
August 07, 2000, 15:07
In the case of the AR series, as you probably know, the gas goes from the bolt key into the carrier and then there are the 3 gas rings on the back of the bolt. (not to insult your intelligence) but my point is, if it flowed directly into the top of the carrier and then out a vent or vents, it seems that you would stand a bigger chance of being affected by it as it leaked out the back of the body cover.
Not trying to flame or insult you, but that is how I see it.

Dano
August 07, 2000, 15:47
Hey Barley! Good to see you chiming in on this.

The FALs are more accurate with the gas off. However, with the gas off -- they're effectively a bolt gun and not a self-loader.

Hey Recce! -- No flame taken.

I know all the answers are not known, we are after all, exploring new territory here.

You did bring up a good point about venting the gas in a safe direction. Much appreciated.

My objective here is to have us work together to improve the breed.

My DSA FAL w/medium contour barrel is as accurate as my Rem-700 bolt gun -- provided the gas is turned off.

My AR10T is also as accurate as the Rem-700 ... IN SEMI-AUTO mode!

Why can't the FAL also do this? I think it can. We just need to put our heads together and make it happen.

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

[This message has been edited by Dano (edited August 07, 2000).]

builder
August 07, 2000, 17:17
Great question Dano. But you've misdirected your question. Asking GP or Derek (no flame intended) is like asking your neighborhood
lawnmover repair shop to ground build a Nascar for the Daytona 500. GP and Derek are self hyped parts assemblers for heavens sake.

Idea sure sounds interesting though.

Just my 02.

Brad's
August 07, 2000, 19:40
Why not pin or thread a fixed Titanium piston to the bolt carrier. It would be light and reduce the shock of the piston hammering on the carrier face. And all the gas venting can be kept out of the receiver body. The new piston could even be hollow, if it works in my drag engine. Then why not? The new type of piston would be less than 30% of the steels weight if it was made solid and about 20% if hollow. A direct gas system would not be overly practical. Think, Titanium, and go play some golf with your FAL. Ping, ping, ping, bang, I win. Brad.

Brad's
August 07, 2000, 19:43
Why not pin or thread a fixed Titanium piston to the bolt carrier. It would be light and reduce the shock of the piston hammering on the carrier face. And all the gas venting can be kept out of the receiver body. The new piston could even be hollow, if it works in my drag engine. Then why not? The new type of piston would be less than 30% of the steels weight if it was made solid and about 20% if hollow. A direct gas system would not be overly practical. Think, Titanium, and go play some golf with your FAL. Ping, ping, ping, bang, I win. Brad.

Dano
August 07, 2000, 20:03
Titanium piston? Hmmmmmmm. I wonder if the reduced mass alone would be enough?

I wonder how much one would cost to manufacture? If it would work, it'd be a drop in upgrade. I like it!

In the mean time, I think I've figured a way to build my direct gas system. If what I have in mind works, it will be both cheap and relatively eazy to do. Key word here is "IF". Give me a few days and I'll let you all know if it worked.

In the mean time, lets keep brainstorming this thing. There's been a lot of good ideas generated already. Lets keep this going!

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

Augdog
August 07, 2000, 20:10
Dano,

I would have to say that is a good idea, and probably be easy enough to design, but it may not help the accuracy much. Looking at my SIG 550's they have gas pistons like the FAL,but they are attached the bolt carriers, and the three I have shot will all group under 1 MOA. The one thing in common in the very accurate semi auto's is a rotating bolt design, as in the AR10T, SIG's and AR's. I think it may have more to do with the lockup of the bolt than the gas piston. There are some very accurate FAL's out there, so it may be in the tolerances and not the design. Maybe you could try 2 different designs?
First gas operated, and then if it don't work try Brads idea, attach the rod to the bolt carrier, by removing the spring and drilling a larger hole in the reciever to be able to assemble it. Of course none of this may help due to the lockup design. (which is great for what it was intended). I do like Brads idea of a lighter piston, maybe we could find someone to make one.(DSA's is hollow but not titanium)
Ok enough of my rambling.

Augdog

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"One man with courage makes a majority."

[This message has been edited by Augdog (edited August 07, 2000).]

recce
August 07, 2000, 20:27
Ok, how about if the vent holes in the carrier were angled towards to muzzle end of the rifle. This would add an extra bit of push backwards, and if you used a Canadian short body cover with stripper guide on it most of the gas should vent before the carrier is completely inside the body cover.

Dano
August 07, 2000, 20:47
Augdog,

At one time I too thought it was because of the bolt/locking mechanism. However, when I shut the gas off and work the bolt manually, groups tighten up dramatically using the same bolt design. Because of this, I really don't think its the bolt design.

If you really want to tighten up your groups, shut off the gas AND remove the piston. You'll be impressed!

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

Augdog
August 07, 2000, 22:15
Dano,

Good point, I have noticed bad verticle stringing on two my FALs. I will try your suggestion next time out. Of course I may not notice any difference, due to the fact that I can't hit the broadside of a barn. http://www.fnfal.com/forums/rolleyes.gif

Augdog.

kev
August 08, 2000, 12:38
First thing, get away from the AR comparison. People call the AR a direct gas action, but it isn't one. The AR has a piston and a cylinder just like most gas guns(M14, FAL, etc), but it's in the carrier. Gas doesn't blow the action open. Gas expands in the cylinder and forces the carrier back. Residue in this tiny place is what causes problems. The design you're suggesting is more like the Swedish Ljungman or Hakim. Direct gas blowing back the carrier. Shouldn't be a problem with fouling; there are no moving parts to foul. You will need to go to the half top-cover or you'll be blowing them off constantly. You will still need a gas regulator at the port. I don't see any reason that it wouldn't work fine. Whether it improves accuracy or not will be the question. I suspect locking, flex, and non-linear cycling mass are bigger problems.

Dano
August 08, 2000, 16:13
Thanks for setting the AR record straight. Every little bit helps.

PS. The FAL already has a built-in gas regulator.

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

kev
August 08, 2000, 18:36
Yes, it does. But I'm not sure you can just use that. Obviously you're going to need to replace the tube or cylinder and I think you'll need to go with a small diameter tube like the one on the AR or Hakim. A large diameter tube will rob all your pressure and without looking at one, I'm unsure whether you can just hook up to the FAL regulator. I didn't mean to turn you off of the idea; just to direct you to the Ljungman system. Take a good look at one of these, and the mods needed to the FAL will be obvious. May take some tinkering, but it may very well work.

[This message has been edited by kev (edited August 08, 2000).]

Dano
August 08, 2000, 19:41
No sense in reinventing the wheel.

Where can I get a look at the Swedish Ljungman gas system, either a photo or a diagram?

My reference books doesn't show the gas system well. It shows the rifle yes, but the gas system no.

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.



[This message has been edited by Dano (edited August 09, 2000).]

Mycroft
August 09, 2000, 16:10
Interesting thread, guys. Regarding the idea of affixing a gas piston to the bolt carrier (a~la AK-47), I recall some debate on the old board over whether or not the bolt "rode" the gas piston as the action closed after each shot. I believe someone pointed out a slow-motion video scene on the AGI videotape that proved that the piston snapped closed before the bolt could strike it. Obviously, that would change if those parts were permanently joined. Would there still be a gas piston spring, or would there just be a stiffer action spring to deal with the mass added to the bolt carrier?

Also, with regard to other designs, wasn't the MAS49-56 a direct gas-impingement semi-auto? There might be some design features there that are worth "assimilating".

M4

Dano
August 09, 2000, 17:25
Well, the first tests have been run and here's what I've learned.

A gas tube can be fabricated relatively easily by cutting the piston head off of a standard FAL piston and drilling a hole through it. It is soft steel and drills rather easily. I press fit a 1/4" seamless stainless tube on the piston and a collet ($0.47) was placed on receiver end of tube (inside handle nut) to prevent tube from reciprocating.(see photos).

The bolt carrier face can be drilled, but is a hardened steel and must be drilled using a cobalt drill bit. A machined flat on top of carrier surface prevented drilling hole deeper than .400" (see photo). Any deeper, and the drill would have broken through the surface.


Modified Bolt Carrier & Gas Tube (Front view)

http://albums.photopoint.com/j/View?u=973743&a=7145445&p=25909579&Sequence=0


Modified Bolt Carrier & Gas Tube (Top view)

http://albums.photopoint.com/j/View?u=973743&a=7145445&p=25909578&Sequence=0


Bolt Carrier Flat

http://albums.photopoint.com/j/View?u=973743&a=7145445&p=25909580&Sequence=0

Did it work, you ask?

With the gas adjustment ring turned to one, bolt would reciprocate about 1/2" or roughly the length of the hole in the bolt carrier face. So yes, it did move the bolt back, however, not enough to eject and rechamber a round.


What about gas blowing back into your face?

Blow back on the shooter was not noticeable, no odor of burnt powder, nothing. As far as shooting, it was much easier on my nose than my AR15.


Did it blow the cover off the receiver?

No. Not even close. From the shooter's point of view it just felt like I was shooting a FAL.

I would appreciate any feedback. Thanks -- Dano

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

[This message has been edited by Dano (edited August 09, 2000).]

kev
August 09, 2000, 18:18
The MAS 49/56 is similar. If I get a chance when I get home this weekend, I'll post some pics of the MAS and Hakim.

recce
August 09, 2000, 20:54
Is there an extension from the "tube" into the carrier? Would this help to stop some gas leaking and direct it all into the carrier? Are the para bolt carriers milled on top?

Yeah I know I doubted it at first, but the more I think about it, the more intersting it is. Good luck on working it out.

Dano
August 09, 2000, 21:37
Yes, the tube protruded through the receiver and into the carrier about 1/2".

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Those who beat their swords into plow shares, usually end up plowing for those who didn't.

L/FN
August 10, 2000, 16:42
Why not mount target sights on the end of the muzzle instead of the gas block? Seems like 8-1/2"s of additional sight radius would make a world of difference!
Look at the sight radius of a M-14 to a FAL!!
I have a HB FAL with a custom, very high cheek rest buttstock. I use a ARMS mnt as a flat top base and have a M-60 sight attached to the mnt with a custom double throw ring lever system. For the tall front sight I built a sight that will lock up or down by a detent system. Up for iron sights and down for transport or when a scope is mnted on the flat top rail.
I currently use a M-60 bipod on the front with custom bipod coupler & muzzle brake, but I'm thinking of going completely M-60 front end with a brake instead of the 60 F-hider.
This would give me a much longer sight radius and make my HB even more accurate than what it is.

kev
August 11, 2000, 20:01
Here's a link to some pics of the Hakim(Egyptian Ljungmann). Action is quite similar to the FAL.

<a href="http://albums.photopoint.com/j/AlbumIndex?u=170089&a=7673096">
See my photos of Hakim Gas System at PhotoPoint</a>

Richard Bird
August 11, 2000, 23:38
Ever notice how the end of the piston rod is internally threaded----maybe for attachment to a modified bolt carrier for some yet unseen special issue FAL based sniper rifle?
Hmmm

Dano
August 12, 2000, 02:12
Kev,

Just got a look at your photos -- Thanks!

The similarities are uncanny, I'm obviously not treading any new ground here.

I can see that this type of gas system is new only to the FAL. That is good news!! Someone else a long time ago was able to make this system work. We don't have to reinvent the wheel after all!

BTW, was the Hakim reliable and robust?

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If guns cause crime... matches cause arson.

kev
August 12, 2000, 04:25
Reliable and robust. The two countries who used this system are Sweden and Egypt, so I think you could say that it probably handles cold and hot, wet and dry, as well as sandy environments. The Egyptians also patterned the Rashid on the Hakim. Basically a down-scaled rifle for the 7.62x39 AK round. Same action.

The rest of the action is very similar to the FAL, with the carrier locking the bolt into a cutout in the bottom of the receiver. The gun is a little abusive to fire due to noise, tho. I don't know how much of this is due to the gas tube aimed at your face and how much due to the muzzle brake. I'm sure the carrier redirects most of the gas up and forward. The Hakims were cheaper than dirt 10 years ago, but now run 3-400. I bought this one as a parts gun from Century for $40 primarily because I wanted this action to try to build an accurate autoloader, thinking like you that this would be one of the best type actions to use. Just haven't gotten to that project yet.

Radio
August 13, 2000, 02:47
Happened to run by the house on my way to Kalif and checked the Files long enough to run into this topic. Richard Bird: gas piston on StG is threaded to screw in a brush for cleaning, IIRC. Dano: very interesting research work there, must be fun to have enough parts kits to afford messing around with one like that. As to getting better accuracy with the gas system in "G", below is Ted II's excellent answer to this question from the old Files:

Question: "Will a FAL be more accurate if the gas block is set to grenade launching position (single shot)? I would assume so since the bolt doesn't move. Has any one tried this?"

Answer by E.M. (Ted) Dannemiller II: "The bolt moves, but not enough to cycle the action! Iíve witnessed MANY a time using grenades, and other times wondering why the damn gas setting was wrong. (Damn, I left it in 'G' again!) A small amount of gas may escape through the port. Not always, but itís unusually evident when gun is cleanest! Understand the gas cycle. The bolt does NOT move until the round has left the barrel. Once de-pressurized, the bolt carrier then unlocks and the bolt moves rearward. The round is travelling at 2600 fps, +/-, and the gas is expanding into an atmospheric volume at the same, but diminishing, rate. The gas then works the piston, which works the carrier, which works the bolt. Timing is everything. I don't believe that 'G' provides any intrinsic gain in accuracy."

If you shot tighter groups in "G", good for you, not saying you're mistaken since I wasn't there, but I don't see how such results have any basis in the physical (rather than psychological) realm. Surely hasn't made a lick of difference in my rifles, just increased the slap!

Better e-mail me with flames, hitting the road again and won't be back for about a week... wouldn't want to miss them. http://www.fnfal.com/forums/wink.gif

--Radio

Dano
August 13, 2000, 14:11
Radio,

There is no doubt that several of my FALs have a vertical stringing problem. You can try it youself, with your own rifle. Just bench rest the rifle, load 10 rounds and then fire a carefully aimed 10-shot string in semi-auto mode. Then shut the gas off and shoot another carefully aimed 10-shot string, you'll see what I'm talking about.

Ted is correct about the bullet being outside the barrel before the piston acts upon the bolt. When you fire your 10 round semi-auto string, you'll find that the first round (the hand jacked round) usually strikes lower than the others. The subsequent rounds will start going vertical. The degree of vertical spread varies with the rifle.

The problem (my theory) is what the piston does to the barrel after the piston returns, not what the piston does while the bullet is in the barrel. With the mass of the piston and the velocity of its return, it is nothing short of whacking the gas block with a hammer-like blow. This is not good for accuracy.

To prevent this problem, we can go down three paths. Remove the piston all together and design a direct gas system, reduce the mass of the piston as well as the spring rates that return the piston, or attach the piston to the bolt carrier. The second option would reduce the blow to the gas block. The third option would eliminate the blow by stopping the piston short of the gas block.

I'm still playing with the direct gas system. I have learned that the Hakim rifle used the same system many years ago (thanks Kev!). I still have work to do in order to get the tubing diameters and pressures just right.

Dan at VOW is thinking about machining a titanium piston that I can experiment with. Until I complete the experiment, I won't know if reducing the mass of the piston will be enough to increase accuracy in semi-auto mode (accuracy is just fine in manual mode). I sincerely hope this path works, as it would be a drop in improvement. Where as the direct gas system requires some modification (drilling)to the bolt carrier.

I have not had a chance to play with the attachment of the piston to the bolt carrier as yet.

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If guns cause crime... matches cause arson.

[This message has been edited by Dano (edited August 13, 2000).]

Brad's
August 15, 2000, 04:06
Well guys, I sure hope that it works. Titanium rules. Please don't forget to send me a Titanium piston for my Israeli FALO, at a deeply discounted price of course. And if you start making the Titanium piston to modified bolt carrier unit, don't forget to send me a 25% cut on all of the sales. :} After all it is my idea and as the conceptualizer I should get a fair share don't you think. I've had several ideas make good money and maybe this time, I'll get something for my trouble. Oh well, maybe this time I'll get the cabin by the lake, instead of the pat on the back. Have fun! E-mail me on your progress. Brad

Brad's
August 15, 2000, 04:09
Well guys, I sure hope that it works. Titanium rules. Please don't forget to send me a Titanium piston for my Israeli FALO, at a deeply discounted price of course. And if you start making the Titanium piston to modified bolt carrier unit, don't forget to send me a 25% cut on all of the sales. :} After all it is my idea and as the conceptualizer I should get a fair share don't you think. I've had several ideas make good money and maybe this time, I'll get something for my trouble. Oh well, maybe this time I'll get the cabin by the lake, instead of the pat on the back. Have fun! E-mail me on your progress. Brad

Dano
August 20, 2000, 09:51
Update:

The direct gas system is workable, but it exceeds my hoped for goal of an easy upgrade (that is -- any garage gunsmith can do it).

It requires building up (by welding) the bolt carrier to allow a larger hole to be drilled to receive the gas pressure. The larger hole is required to multiply the force of the gas, much like blowing air into a parachute. It also requires a modification to the sheet metal dust cover for clearance.

While all of this can be done,it effectively takes the modification out of the "easy" and "cost effective" upgrade category.

I am now looking to see if reducing the mass of the piston and reducing the piston spring rates will have a postive effect on accuracy. This should reduce the blow the piston imparts on the gas block.

Dan at VOW (thanks Dan!)is fabricating a titanium piston for me to experiment with. As soon as Dan is able to get it to me, I'll run a few tests and report back with the results.

But for now, if you want/need the best accuracy from your FAL, turn off the gas and hand jack the rounds in.

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If guns cause crime... matches cause arson.

ErikAndersson
August 20, 2000, 22:56
The FAL does not require the piston spring at all to work.It's main purpose is to delay the unlocking of the action and to hold the piston out of the way when the action is open.I have had to use the rifle without one before and it worked just fine.I'm not sure what problems would arise in the long term,but it does work without the piston spring.

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Cut the blue wire! Erik

Dano
August 21, 2000, 07:22
I am aware that the spring is not necessary to cycle the bolt. But allowing the bolt to return the piston is not a good idea.

The piston is only in contact with the bolt carrier for about an inch. After that the bolt continues backward and the piston slows creating an ever widening gap between the two. The down side of letting the bolt return the piston is the bolt wallops the piston HARD on the return trip sending it on to a HARD impact with the gas-block. The whole operation is somewhat like a hammer striking a nail.

The reason for the lighter spring is to LIGHTLY return the piston before the bolt carrier can impact it. This will lessen the impact of the piston on the gas-block.

If you would like see the effect that the piston impact has on the barrel pull the piston back about 1.5-2" against its spring and let it go (this is easier to do on a FAL with a short gas-tube). On a FAL with a free-float handguard, the barrel "Rings" like a bell. This type of uncontrolled barrel vibration is not good for accuracy.

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If guns cause crime... matches cause arson.

[This message has been edited by Dano (edited August 21, 2000).]

W.E.G.
August 02, 2001, 19:24
moved to FAQ

Stranger
August 03, 2001, 12:29
Concerning verticle stringing, did anyone mention the effect of heat on the barrel? A "cold" barrel is going to shoot differently than a "hot" barrel. It would be an interesting experiment to keep a barrel at a constant temperature and see what effect it has on accuracy. When the barrel whips and bulges around the bullet, there must to be pressure points around where the barrel is held in place. Any change in temperature would affect the rate/direction at which the "deformations" occur. The fact that some rifles show stringing and others don't tells me that it isn't the gas system, but something in how the barrel accepts the bullet. Maybe there is less stress in some of the barrels and connections.

Why is shooting with the gas open more accurate than with it closed? The first thing I notice is that I am much more careful about shot placement when I have to hand cycle the rifle compared to when it is in semi-auto mode. If you have the rifle in a stand and it still strings vertically, then it definitely isn't due to operator error. My point is that hand cycling usually translates into more time in between shots. Maybe the longer time between shots allows the barrel to cool to the point that heat is removed from the equation. Anyone have any comments on this? What are your experiences? Do you make sure that you shoot at exactly the same rate when hand cycling as when letting the rifle do the work?

[ August 03, 2001: Message edited by: Stranger ]

EMDII
August 07, 2001, 14:32
Lessee:

- weld up the gas block exhaust
- fab a new gas tube w/ no exhaust ports
- oops, go back and fab full-length gas tube (clang)
- Seal the gas tube to the keyway in the BC headpiece

BBBZZZZTTTTT!

Monsieur Saive's succesors at FN-Herstal developed a 5.56x45mm FAL in light of the success of the M-16. They cvalled it the CAL, and then the FNC. Why do you suppose they used the same piston, and lots of other interchangeable 7.62x51mm parts, instead of selecting the Stoner gas system?

Perhaps they're pretty bright Engineers, after all?
:p