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SMALLEST GAS PORT SIZE??

6K views 48 replies 8 participants last post by  1biggun 
#1 ·
I am starting on my 17-223 varmit build #2(waiting for reamer) It has occured to me that the gas port hole on a 223 is almost as big as the .17 bore.
What is the smallest gas port hole any one has gotten away with on a 223 build?? I am thinking of trying a custom GB with tighter piston to block clearances and possably having the piston run an inch or so in this block with tighter clearances to try to get the bolth my 223 and the soon to be done 17-223 to cycle with a smaller hole. was also thinking of a lightest possable recoil spring. was also considering a smaller piston as well thinking it would take less volume to move it. was going to try one the same as a FAL

I also need a few AMD 65 pistons to play with If anyone has a few originals laying around CHEAP I could use a few to play with. I gave mine away a year or so ago.
 
#2 ·
(cross-posting from GNA)

Biggun, all I can give you on this is the size for .223 builds (based on my Saiga and Galil barrels) -



http://pookieweb.dyndns.org:61129/AK/docs/dimensions.htm



5.56x45 Saiga 90 degree .067" # 51 bit 11.9063" (11 29/32") / 302.420mm




That's the smallest from the "factory" stuff. Since I have only a good idea what a 17-223 is, I wonder if there are any AR builds in a similar caliber? 17 hornet or HMR or whatever it is called? What would that use? I wonder if Varmint Al has anything on his web page.



.177 - .067 = .011 so you would still have "enough" barrel around the hole, IMO.
 
#3 ·
the gas port hole is about .110" smaller than the bore. the main thing to consider is the distance the gas port is from the muzzle. the force delivered to the piston is a combination of pressure and time. if the .17 round delivers simillar chamber pressures and the gas port is far enough back it should work fine with a sdt gas system.
 
#4 ·
I've always wondered -- Once it unlocks is it the recoil and barrel pressure that drives the carrier back (ala blowback) or is it the pressure in the gas tube?

Figuring the overkill in function design, I wonder if the little .070" hole would operate a 7.62. Next one drilled I may try that.
 
#5 ·
It is basically the momentum from the initial force of the gas. the gas tube on an ak is vented where the tube meets the gas block. on an ar the gas tube is only into the bolt carrier a short distance. on an m1 carbine it is just the push from the gas piston that cycles the gun.
there are tremendous forces at work, considering the head of an ak piston is about .500" the surface area is about .2 squ in. and chamber pressure is about 50,000 cup you have about 10,000 cup of pressure acting on the face of the piston for a couple of milliseconds.
 
#7 ·
thanks for the input. I want to keep the port as small as possable because accuracy is the main goal here. I am afraid that the relativly large port compared to the very small bullet will negitivly affect the accuracy, and possably shear lead off the bullet.
I am buildng a gas block for my 223 varmit build that will use a a tighter fit between the piston and the block for about 2 inches, after the piston travels back the 2 inches it will reach the tube and also have some venting. I am going to start with a very small hole and work up until it cycles. if it works out I will duplicate it on the 17-223. my thinking is the tighter fit will allow it to work with a smaller hole because it will initally work due to pressure and not the force of the blast. once it has started to move the venting will keep it from shoving the carrier out the back of the gun.( I will deffinatly be string testing this set up. the last thing I want is a full 60,000- 100,0000 psi on the piston face) I am also incorperate a shut off valve in the block of some sort most likely a cross pin through the hole.
I figure it is best to work it all out on the cheaper and more worn barrel first. I have a fair amont of money invested in the stainless 17-223 barrel and I want to get it right the first time. It looks like the pressures in the 17-223 will be around 50,000 psi. I have been warned to keep the pressures and velocity down on the .17 to prevent barrel erosion and rapid fouling, so it may be in the 45,000
originaly I was going to use a adjustable FAL style block but after thinking about it it would require an even larger hole in order to give adjustabilty as they work by bleeding off excesive cases when hotter loads are used.
 
#8 ·
If I'm following what's being said;

The piston has barely moved (but is at max velocity) when the barrel is clear and pressures have dropped to nothing.

The little movement has not even unlocked the bolt by the time the bullet is out.

The remainder of extraction, ejection, and spring compression is done by the carrier momentum.

Thank you gentlemen.
 
#9 ·
I am starting to be concerned that on my 26" barrel with over 16" in front of the Gas block that it may unlock to soon. I am starting to rethink using a amd length piston and thinking a longer than normal one may be in order. my original reason for going with a short piston was mainly looks and trying to balance the gun out some as it is pretty nose heavy now. I was also thinking there may be some accuracy gains from having the bloch mounted further back in the heaveir prt of the barrel. My next question is on the very long barreled AK style weapons is they all use a standard length piston CORRECT??
 
#11 ·
The longer stuff needs a longer piston or carrier. I just put the gas block on the 308 barrel with the PSL gas tube and with a std Yugo carrier/piston, the piston is about 3 inches too short.
 
#12 ·
Gunter how long is the original barrel?? The other thing That may come into play in all of this is that the 17-223 has a MV of 4000+fps compared to the 2300 of the 7.62x39 this means that the bullet will be out of the barrel sooner almost twice as soon. this kind of throws things the opposite as far as lock time, making me think that a shorter piston may work. I think the gas tube and piston on a 223 sagia and the 223 SARs are standard length so I am going to go with a standard length piston on bolth builds.I am still going to try the tighter fitting piston riding in a custom gas block for 1.75 inches this will be acomplished by using a AMD gas tube and a standard length piston that protrudes 1.75 out of the amd tube. the the protruding amount of piston will be fitted into a custom gas block were it will run tighter clearances than normal hopefuly working with a smaller gas port. I relize that this may affect the reliabilty of the weapon after a lot of shooting due to carbon build up but it gets cleaned about every 25 shots anyways I am also going to to have clearance rfrom the tube to the block so the barrel will be completly floating still. this all needs to be done plus the entire 17-223 build (still no reamer) by the end of the month as that is when I take it to South Dakota. I have been told that one of my buddies has a new DPMS AR 15 varmit build that he plans to out do me with (I am guessing the new 204 Ruger). we will se about that
 
#13 ·


The barrel is the same, just taken at different heights which makes it look longer in the top insert.

I think the barrel is around 22 inches now without the brake. I would have to measure it again. Shows the size next to a std Romy.

Sorry for the crappy pic. Had to take it indoors.
 
#14 ·
Yes I'm with Winn - I think the gas port location may need to be worked through. The issue will be the "burn time" if the port is too close to the chamber. I think.

It could be reasoned logically that the close to the chamber, the more gas will bleed away from the bullet while in the barrel. That, I think, translates to lower muzzle velocity. That could probably affect accuracy in the long-distance area.

Now, I don't think you'd get a squib effect by having a "short" piston... I really don't think that would behave that way. If it did, then your load is REALLY light!

Turn this around. The impulse on a Dragunov, PSL, and other heavy-caliber "squad sniper rifles" is being pushed by a significantly higher load of gunpowder in the 7.62x54r and therefore it really needs a longer piston if you follow the logic that, at a certain point, the pressure in the barrel will be at the optimal point in-between high-pressure beating of the gun, and low-pressure poor performance (as if the gas block were at the end of the barrel).

Thus, a longer piston design may be counter-productive for a smaller round. Consider yet again the AKS-74U design. That cool cone flash hider has a purpose - it is an expansion chamber that serves as a muzzle BOOSTER which helps maintain velocity and permits the AKSU to cycle properly.

I wouldn't even consider AR designs since they don't use a piston. :/


The 24" RPK barrels are a full 8" longer than the 16" AK barrels. The muzzle velocity is noticeably different on my RPK. In fact, it is noticably different on my Chinese NHM-91 with its 20" barrel... amazing that 4" can make that much difference... however the point is that the RPK has the same piston as an AK.

An AMD-65, with its 12" barrel, has a piston that is only slightly shorter than a normal AK. 120MM IIRC. That puts us where? 16"-12" = 4", so there is a diminishing return there somewhere.

There is also a point where the barrel "whip" harmonics encountered during the firing cycle would let you optimally place the gas block to minimize barrel oscillations, however this is conducive only to accuracy and not function, if one were to follow the logic.



So where does this leave you? I'm not sure! :(

If I were to follow my logic (perhaps flawed since it only considers function!), then I would imitate an RPK-74 design and use a normal piston. An RPK-74 is probably closest to your caliber as can be had. The Czechs use 5.56 in their VZ-2000 (I forget - is it actually called a VZ-2000?) which is also a close model for you to follow. Their RPK version has a standard piston vs. the "standard" model. Food for thought.

HTH,
- Jerry
 
#15 · (Edited)
Not to highjack 1biggun's thread.

You guys might like to check this video. Not about AR vs AK, but for the slo-mo AK footage. If you look close you will even see a left handed AK.

[video=youtube;G6BpI3xD6h0]http://www.youtube.com/watch?v=G6BpI3xD6h0"]http://www.youtube.com/watch?v=G6BpI3xD6h0[/video]

Another thing to ponder is that it is trying to push a large amount of powder/gas down a tiny hole. The pressure curve would be different than say a 7.62x39.

I just copied the PSL design since the 7.62x54R and the 7.62 NATO are close.
 
#16 ·
I have seen that video before and was thinking about it last night ( I need to get a life) it occured to me that since there is basicly no barrel in front of gas block on my AMD 65 that that there is no whip between the port and the end of the barrel since there is basicaly no barrel. this may account as to why my AMD 65 is the most accurate AK I have in 7.62x39 with a factory barrel. .75 -1.00 groups at 100 yards pmc ammo and a big scope. the gas tube is ridigidly clamped to the barrel and it takes a hammer to close the gas tube latch the reason is that the scope mount is on top of the tube and it must not move at all. I think that it is possable that the tube is actually reinforcing the barrel and reducing the harmonics and whip. I had heard of guys claiming that 16" barrel AK,s shot better with the tube clamped to the barrel. before seeing the slow mo photos I dismissed it, but now it has me rethinking how to acursize a light barreld AK.

as far as my 223 and 17-223 the barrel is aprox 1.00" were the gas block will go so I am hoping that barrel harmonics will not be to much of an issue. I have been giving this a lot of thought and I have ideas from shorter pistons to running a flexiable tube from the muzzel brake back to were the piston ends. I guess with out a degree in some type of enginering (I got a D in high scool remeadial math) the only thing I can do is make an educated guess and drill a smaller than normal hole and start from there. I believe that in a 223 all the powder is burnt at aprox 20-22" and that no real gains in velocity are made by having a longer barrel. I am going to go with a standard length piston. since I have to make the GAS block anyway to fit the 1" barrel, I am going to have it ride in the block for aproximently 2" in a tighter than normal bore this should allow for a smaller gas port. If it dont work I can always open it up and port it to act like a standard system. I am going to have a gas shut off as well this way I can compare the accuracy of SA versus cycle by hand. I have acess to a chronigraph and will be able to compare velocity as well.
My main concern right now it that the bolt will unlock to soon due to the long barrel and blow up. the 17-223 has a a 22" barrel so hopefuly all this timing will not be an issue.
another way to do this would be buy 10 barrels and and keep drilling untill I get the best accuracy! All I know is on the AR15 build I am planing I am buying the stuff already done with no guess work.
 
#17 ·
for the time factor consider that a bullet going 4,000 fps will go 1' in .00025 seconds, one going 2,300 fps will travel 1' in about .00045 second, if the chamber pressure is the same I don't think the time factor will mean much in the transfer of energy, as long as sufficient energy is imparted on the piston to operate the action. don't forget the energy is only on the piston when it is in the gas block.

as afar as bbl length vs velocity I think I remember readin that there is a 10% loss in velocity for every inch of bbl less than the optimal length the load was designed for
 
#18 ·
I remember reading that there is a 10% loss in velocity for every inch of bbl less than the optimal length the load was designed for
Thats what I recall as well. but it might have been 100 fps per inch. it is hard to bellive that a 12" amd 65 would be 40% slower than a 16 inch AK. I think it was 300-400 fps slower how ever. I need to get on a thompson /encore sight and see what those pistols show. I thing optimum in a 223 is aprox 22" for most loads.

don't forget the energy is only on the piston when it is in the gas block.
this my thinking as well that is why I think having the piston in a tighter bore for a longer distance will work with a smaller gas port. once it travels the 2" in the bore it will be ported were it enters the tube. I have some concerns that if I get the piston bore clearance to tight it may build up high pressure and break the block.
I hope to shoot it this week end one last time with out the gas bloc to fine tune my hand loads. then I plan on tearing the gun down to fit the gas block and drill the port hope to have a answer to some of this in a few weeks. also adding a adjustable comb and a adjustable rear rest to the fiber force stock. working on a shell catcher and bolt hold open selector as well. this time I will get some good pictures to post.
 
#19 ·
one thing to remember is that even though pressure chamber might be 50k CUP it will be diminished somewhat by the time it reaches your gas port. I dont remember the exact numbers but I read an article where they had ported diff AR bbls at diff distances from the chamber and the pressure had dropped by drasticly by the time the bullet reached the gas port on a CAR length bbl.

I have a 16" galil with the gas port 6" from the breach face (16" micro galil) that works like a dream with a .040 gas port. If you happen to drill a hole slightly to large you can always turn the head of the gas piston down slightly to creates some blow by and soften the unlocking, same principle as adjustable gas systems that vent excess gas . my .02
 
#20 ·
You are correct on the AMD muzzle velocity - only about 400 FPS slower. The latest issue of G&A's Combat Arms annual lists the AK-102 from Arsenal (called an SLR-106 or something) has about the same loss of velocity - about 200-400 FPS depending on ammo and use of the muzzle booster.


I really don't think you are going to have a problem. Much of this is academic since the AR platform can shoot so accurately with its gas-feed system, I don't see any problem with where you put it. I believe the impulse would be a problem only if you put it close to the muzzle, like an AKSU design. The AK-102/4/5 SBR design uses a muzzle booster as well. However, both systems can be fired without the booster in place, suggesting that in all but the worst of conditions, you should have no problem with cycles.

Another thought, was that the K-Var 16" barrel AKSU kits have the gas block at the normal AKSU spot (closer to the chamber), and the 9x39 rifles based on the AKSU also have the gas port in the "normal" AKSU location at slightly over the 6" mark. That puts 10" of barrel - 2/3 total length - ahead of the gas port.

I think it will work :D
 
#21 ·
17-223 reamer came today YEA. pilot does not fit into barrel BOO. removable pilot coming YEA. I have been dealing with pacific tool and guage for this and the service so far is good. the pilot issue is not there fault. I should have micd my barrel before ordering. spoke to the owner seems like a good guy very help full answerd some questions on 1.5"x308 barnes and others.
 
#22 ·
I have a 16" galil with the gas port 6" from the breach face (16" micro galil) that works like a dream with a .040 gas port.
.040 this is more to my liking.

if I was not going to be cleaning the 17-223 some thing like every 10 shots I would avoid the small hole stuff as it may foul shut but I really am trying to get this thing be as accurate as possable no mattter how much F!@# around i have to do to keep it cycling. I am thinking an acess hole for cleaning the port would be a good idea. Of course now that I have the reamer There will be a 17-223 light weight version and a pistol as well. I have no good reason for building the pistol other than it will be the only one on earth.
 
#25 ·
By hand?!? That sounds odd. I had thought that with the reamer in the tail chuck and the barrel in the head stock, that was the only way to keep it centered.

I had assumed that a lathe was mandatory for reaming, and that's why I'm so buggered by my lathe being off-kilter.

I wonder what he uses to keep it straight? A really long pilot? How else could you keep it from wobbling "by hand"? Hrm...
 
#26 ·
I am no expert here in fact far from it. but I see that most guys use a floating reamer. so I can only guess that the reamer centers its self in the bore. It seams that I have read were some guys ream the barrel buy hand with no lath at all and spin the reamer by hand. May be an expert will answer here. I plan on reading up on all this this week.
 
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