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Will this blow me up? 7.62x25mm barrel

3K views 37 replies 15 participants last post by  Bullpup 
#1 ·


I've been working on this project on an off for a few months. Its a 7.62x25mm chambered AK front end. Its a 16" barrel with a very short piston (about 1.5") on a regular carrier. I haven't put a gas port in yet. The gas port will be just about 7" from the breech end, if you subtract the 1" approx case length, then 6" from the case mouth. I'm starting to worry that if the bullet has not left the barrel by the time the bolt rotates and begins to extract then I will get a case head rupture :death:. I really don't want to blow this up.

I've seen a few other builds, notably PBB's build here, but that build used an AMD length piston and 5/32" gas port (.156") and a carrier which has been significantly lightened, recoil spring reduced. the other reference point I have is Gnarlytoe's build:
The AK Forum :: Log in
which uses a krink piston and an 20" barrel and a .205" gas port, recoil spring reduced. I can't seem to find a reference for the length of a krinkov piston, but it's longer than 1.5" by appearance anyway.

Is there a calculation for timing an ak barrel? Chamber pressure, port size, port distance from breech, carrier weight, and barrel overall length? there are a ton of variables! Also, bullet acceleration: if a tok round is going 1400fps from a 4" pistol barrel, then at the 6" gas port mark (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel. The acceleration of the carrier backwards is going to be many times slower, + the spring's resistance, but I have no idea what the carrier weight is or what the pressure is at 6" mark, or how to factor in the gas port size. :death:

then again I might just be making myself crazy.
 
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#2 ·
thats some pretty good figuring there...

first thing to throw in to you is that 1400fps is a LOW number on 7.62x25 ammo.. most of it is closer to the 1600+ range to start with on surplus stuff.

AZHONKEY is a guy who does a lot of this type work and his stuff is excellent and functional you may wanna look up some of his work. but im not sure if he has done the gas piston set up or if all his stuff was just blowback operation.
 
#4 ·
Is there a calculation for timing an ak barrel? Chamber pressure, port size, port distance from breech, carrier weight, and barrel overall length? there are a ton of variables! Also, bullet acceleration: if a tok round is going 1400fps from a 4" pistol barrel, then at the 6" gas port mark (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel. The acceleration of the carrier backwards is going to be many times slower, + the spring's resistance, but I have no idea what the carrier weight is or what the pressure is at 6" mark, or how to factor in the gas port size.
HMMM given the number of differant builds I have seen with pistols that have barrels less than 3" long to my varmit AK with a 26" barrel there seems to be no known fromula for barrel timing. your concerns are valid. MY gut tells me that if a 3" barrel on a 7.62x39 dont blow up then your build will be ok. I doubt there is anyone who will say absolutly for sure. sorry I cant be more helpfull as mentione above AZHONKEY HAS PLAYED WITH THIS STUFF AN BUNCH i WOULD TALKE THAT ADVICE AND SEE WHAT HIS GUT TELLS HIM.
 
#6 ·
The issue isn't barrel length, but gas port distance from the muzzle.
I don't think the port distance from the muzzle is a problem either, I'd worry about the port size. To small of a port and it won't cycle properly (recoil spring strength will factor in this too), to large of a port will let to much gas bled off and might not provide enough pressure for the bullet to exit the barrel.
I wouldn't worry about FPS but how much pressure you'll need to cycle the bolt, as long as you just bleed off enough gas to cycle the bolt the X25 round will provide more then enough to get the bullet clear of the barrel.
I know this is a simple reply, but I think your looking at it in a complexed way and making it harder on yourself.

So the big questions are; Port size? & Recoil spring strength?
 
#7 ·
I don't know if this helps at all but I built an ak pistol in 7.62x39 based on one of slamfire's designs and used a 13/64 bit to drill the gas port hole like he recomended.I wanted to see how much backpressure could be lost and still cycle the bolt so I took the gas tube off completely and fired 2 rounds.The bolt cycled fine even without it.
 
#8 ·
THE TUBE HAS NOTHING TO DO WITH IT. IT ALL HAPPENS WHEN THE PISTON IS STILL IN THE GAS BLOCK. THE BLAST OF THE GAS BLOWS BACK THE PISTON. IT DOSENT PUSH IT BACK WITH A SEALED PENUMATIC TYPE DEAL.

AS FAR AS BULL PUPS CONCERNS, HE HAS A VALID POINT THE SLOWER BULLET IN A LONGER BARREL WITH A VERY SHORT PISTON "MIGHT" BE A ISSUE. IF THE BULLET IS STILL IN THE BARREL WHEN THE ACTION UNLOCKS THEN THE BOLT WILL SEE ALMOST THE FULL CHAMBER PRESSURE. iN THE CASE OF A AK IT LIKELY WONT BE THE END OF THE WORLD AS THE CARRER IS LIKELY HEAVY ENOUGH FOR A BLOW BACK ANY WAYS (DO NOT QUOTE ME ON THIS) A GOOD EXAMPLE IS A SHOOT GUN SHOOTING A SLUG WITH A LONG BARREL THE SLUG HAS NOT LEFT THE BARREL BEFORE THE RECIOL IS FELT AND THE BARREL RISES THROWUNG OFF AIM. hORNADY HAS DESIGNED A ROUND FOR USE IN A SHORTER BARREL OF ALL THINGS THAT IS MORE ACCURAET BECAUSE THE SHORTER BARREL ALLOWS THE BULLET TO BE OUT OF THE BARREL BEFORE THE MUZZEL RISES THUS IT IS BETTER ON TARGET. 1400 fps IS 1000 FPS SLOWER THAN A 7.62X39 SO THERE IS A POSSABLITY THAT THE PISTION MIGHT RELEASE THE ACTION WHILE THERE IS STILL FULL PRESSURE IN THE BARREL. LIKELY THE ONLY TRUE WAY TO TEST IT IS A LONG STRING. I SEE A LOT MENTIONED ABOUT BARREL TIMING IN REGARD TO AR,S MAYBE THOSE EXPERTS COULD SHED SOME LIGHT ON DITANCE FROM BREACH TO BARREL LEGNTH??? WHEN i DID MY VARMIT BUILD i WANTED A VERY SHORT PISTON AS WELL TO HELP BALANCE OUT THE GUN BUT TALKED MY SELF OUT OF IT.
 
#9 ·
hey bub why are you yelling?

are you mad at someone?

theres nothing wrong with the point your making.

yes x25 is slower than x39. its also about 50 grains lighter...

and yes we all know that blow back AK's work just fine with 9mm & x25...
 
#10 ·
Your barrel and carrier are very similar to the one I am working on, but I am ahead of you in construction. It is a gas operated 7.62x25 based in the 2XMT mag well. I am using a 10" PPSH 41 barrel with the necessary sleeving to fit it up to a AMD-65 cut barrel kit. My piston extends only one inch past the bolt carrier and I have removed 1" of the bolt carrier front tube section. That puts the gas port about 4" to 5" in front of the chambered round.

I have cut 2" off of the recoil spring and made shorter wire retaining loops for the recoil assembly. This allows the recoil assemble to fit in the shortened bolt carrier at full recoil.

I made my own hammer spring out of 0.045" music wire because the stock spring is too strong. It is modeled after the stock hammer spring, but I cut off one leg to reduce the pressure even more.

Yes it does fire without blowing up! It strips a round from the PPSH drum and feeds it into the chamber pretty good when the bolt carrier is pulled back and released. Every round chambered so far (about 20) has fired inspite of the weaker hammer spring. I started the gas port at 0.125" and have opened it up to 0.1718" in the quest to cycle the action.
It has fully cycled the action once! :rockin: I am now making a new gas piston for it with tighter clearances. The current piston is 0.009" smaller than the ID of the gas block. That is about standard AK clearances.

No pictures yet to post. I am waiting to acheive more reliable feeding before posting a detailed photo spread.

Good Luck!
 
#11 ·
I am now making a new gas piston for it with tighter clearances. The current piston is 0.009" smaller than the ID of the gas block. That is about standard AK clearances.
Good Luck!
Just a thought. Could you add O-rings to the piston head in order to get a tighter fit? I always thought those recessed looked perfect for slipping in o-rings.
 
#12 ·
I thought about trying o-rings for a single test firing and I may still try it tonight. It might demonstrate to me if the tighter piston I am making will do the trick or I need a bigger gas port. The o-ring ought to hold up for one shot at least!

I am working on the tighter gas piston because I would need to buy more aircraft extension drill bits (12" long) at $3.00 each to enlarge the gas port. The gas block is so close to the rear sight block that the drill chuck hits the rear sight block even with 6" long drill bits.
 
#14 ·
Since my gas piston is so short (1") I couldn't plan on a "long stroke" gas operation. I can only get the standard short push of the piston inside the gas block. My gas tube is 4130 tubing that is sized big enough in back for the bolt carrier OD and then it tapers to a smaller size in front to guide the gas piston into the gas block opening. I salvaged and used the rear ring of the AMD65 gas block to add the locking surfaces of the stock system to my custom gas tube.

I hope tightening up the gas piston/gas block clearances will bump up the recoil impulse without affecting reliability.
 
#15 ·
I salvaged and used the rear ring of the AMD65 gas block to add the locking surfaces of the stock system to my custom gas tube.
When you say "rear ring of the AMD65 gas block" are you refering to the gas tube? thats what it sounds like. I'd love to see some pics of your build SW44, and I appreciate the additional data.

I'm begining to think that there are way too many varriables to really acurately calculate what will and won't work. Unfortunately, with all the data, pretty much nothing is remaining constant (barrel length, port distance, port diameter, spring length, carrier weight, ammuniton).

I think that If I start with a gas port which is really small and work my way up, I will probably be ok. I need the smallest possible port that will cycle the action reliably. The closer that port is to the breech, the more sensitve the whole system will be to changes.
 
#16 · (Edited)
I did mean to say "the rear of the gas tube" and not " the rear of the gas block". Its the thingy with the tab at the bottom and angled surface at the top so that the whatchamacallit can lock down the gas tube. There.. clear as mud.:D

I am working through a rough list of detail work to get mine cycling.
1. I worked out any resistance felt when sliding the bolt carrier and bolt back and forth.
2. Reworked the hammer face to allow the AK74 carrier to pass over the AMD hammer. (It was catching on the bottom front of the carrier after the bolt had passed over the hammer on its way forward.) The same mods gave the bolt slightly more leverage when recocking the hammer.
3. Made new lighter hammer springs. I also made a new spring to reset the trigger since the lighter hammer springs have the effect of lightening the force on the back of the trigger.
4. Shortened the recoil spring by 2". This compensated for the extra force the recoil spring was exerting in the 1" shorter bolt carrier tube.
5. Opened up the gas port in stages from 0.125" to 0.156" to 0.172".
6. Now I am tightening the piston to gas block clearance.

The last 3 steps were all accompanied by test firing.

I am getting close and it is getting exciting. I'll start my own thread when I get to the point of pictures.
 
#17 ·
hey bub why are you yelling?
not yelling just have bad eye strain at the end of the day. cant see the letters very well and I might be a little drunk as well. :)

As far as adding orings and extra tight piston to tubes or gas blocks I played with that a bunch. my 223 varmit build has a custom GB were the piston rides in it for around 2" in a pretty snug bore. I was unable to get under a .075 port to cycle. I was attemting to run as small of a port as possable in hopes of accuracy gains. I felt I gained very little, possably was able to run the port .005 smaller than normal. I started at .040 and workd up in .005 incriments after I got the action to partally move at around .055" port. this is with a very smooth action polished and reprofiled hammer ect. I would be very carefull with very tight tubes ect. if the action were to jam or a case were to stick you are putting around 45,000 psi on the gas block, piston ect. some thing might give. I doubt a O ring would hold up to the heat and grime and pressure very long. I considerd cast iron piston rings but decided if a
.002 piston to bore fit didnt really help then nothing will.

heres a pic of the gas block I did. the tube is a short AMD 65 but the piston is full length rommy reaching past the tube into the gas block. this allowed for a snug fit in the block its self aprox .002 . I have had wolf ammo stick when the chamber gets really dirty. and when that happens I can tell you that the block sees some pretty high pressure. I get gas leaks between the barrel and block. not sure what the actuall pressure in the block was but the pressuer in the barrel is at over 50,000 PSI a sealed piston and up jamed up action must see a least close to that momentarily if nothing moves.



yea thats some rust after 1000 rounds on a damp day. it was initally cold blued but it didnt hold up. all better now.
 
#19 ·
not yelling just have bad eye strain at the end of the day. cant see the letters very well and I might be a little drunk as well. :)
If you're running Microsoft Internet Explorer, look under the "view" menu at the top, should have a "Text Size" option. Run it up a size or two and you should be able to see it better. No guarantees if you're drunk though...... :stretcher
 
#18 ·
I weighed a full length AK74 carrier with piston and bolt, and got about 1lb. Not a very exact scale, but for arguments sake lets say I am way off an it weighs 1/2 lb. 7,000 grains to a LB, so 3,500 grains for the carrier assembly as a very conservative measure. the pressure upon reaching the 6" mark on the barrel is propelling an 85 gr bullet to 1400 fps (also conservative measurement). (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel.) the carrier is (3,500/85= 41.1) times heavier than the bullet. that same pressure would move the carrier 41 times slower than the bullet is moving. The carier travels backwards about 1/4" before begining to unlock the bolt. So the bullet will travel the remaining 11" in 1/1518th of a second. The carrier will travel (1400fps /41= 31.1) at 31.14 FPS, or (31x12"=372" per second) and so will cover 1/4" in (372" per second / 4 = 93) or 1/93 of a second. The fractions are really confusing, so let me convert them.

1/93= 0.01075 seconds for the carrier to unlock (1/4" travel)
1/1510= 0.00066 seconds for the bullet to leave the muzzle (11" travel)

or in laymans terms, the bullet will be long freaking gone by the time the bolt unlocks. Unless my math is complete bunk, it would take a much much lighter carrier or a much slower bullet to get a case head rupture because of early extraction. I used conservative measurements and didn't even factor in acceleration of the carrier, the hammer (which will be resting against the bolt) or hammer spring, or the recoil spring. It seems like IF my math is correct, that I am well within the safe zone.
 
#21 ·
or in laymans terms, the bullet will be long freaking gone by the time the bolt unlocks.
IMHO that's right.

In addition, it's not simply the pressure but the volume of gas that does the work; limiting volume through the gas port substantially slows the operation unlocking the bolt.
 
#22 ·
Id love to be able to reduce the port size some how. on my 17-223 the port is half the sizes of the of the bore. One thing I havent played with is differant piston diamiters. perhaps a much larger or smaller pistion would help. in a non vented tight bore it definitly make a differance.
 
#23 ·
About half an hour after I made my post I realized the calculation I used was flawed and really was intended for blowback guns, since it doesnt take into account the difference in how the pressure will push a .30 cal dia bullet vs a larger dia piston head, or how the gas port comes into play. In reality though I still think I'm going to be OK.
 
#24 ·
you need a test string. and a tree. (or go to the range and say hey anybody want to shoot a ak47? LOL ) Im sure you will be fine but string fire it of course. I do that on all my builds. even the matching number and headspaced stuff. you just never know. an post some pics
 
#25 ·
Guys -

Just stumbled across this thread. Some of you have probably seen my little BF carbine attempt in progress, also gas operated in x25.

The port is .169" located 7" from breech. Biggun I somewhat disagree that the initial impulse in the gas block is ALL that drives the piston/BC. However, I once saw a video of some guy in Europe firing his AK WITH NO GAS TUBE and it did cycle (if anyone has the link to that video I'd sure appreciate it). I still believe there is some pneumatic effect inside the tube. Could be wrong though...anyway I'm running a tight sealed gas tube (could hear a pneumatic pop before the port was drilled) and AMD length piston (yields a 5" sealed stroke) with NO snout on the BC. It cycles and re-cocks the hammer, and overcomes the (weakened/shortened) disconnector spring when the trigger is held back. So it's very close. If it were mag fed, it would be finished. Call me eccentric or whatever but I want it to cycle a beltfeed mech in the topcover. I don't know if it will or even can but I'm trying. I also believe that IF the bullet (85g) were heavier, there'd be more intertia and stronger gas pressure. Just my thoughts - I'm here to learn, and share what I have learned -

SW44 - With no snout on the BC, my pressure stoke is tightly contained for a full 5". Also added quite a bit of weight to the end of the hammer (different setup) so even with a weaker hammer spring, it has enough inertia to fire

Let us know how your builds are progressing and what solutions you're coming up with

Steve in El Paso
 
#26 ·
The port is .169" located 7" from breech. Biggun I somewhat disagree that the initial impulse in the gas block is ALL that drives the piston/BC. However, I once saw a video of some guy in Europe firing his AK WITH NO GAS TUBE and it did cycle (if anyone has the link to that video I'd sure appreciate it). I still believe there is some pneumatic effect inside the tube. Could be wrong though...anyway I'm running a tight sealed gas tube (could hear a pneumatic pop before the port was drilled) and AMD length piston (yields a 5" sealed stroke) with NO snout on the BC. It cycles and re-cocks the hammer, and overcomes the (weakened/shortened) disconnector spring when the trigger is held back. So it's very close. If it were mag fed, it would be finished. Call me eccentric or whatever but I want it to cycle a beltfeed mech in the topcover. I don't know if it will or even can but I'm trying. I also believe that IF the bullet (85g) were heavier, there'd be more intertia and stronger gas pressure. Just my thoughts - I'm here to learn, and share what I have learned


HMM this is a good discussion.
like I said I didnt acheve much of an improvement, with a tight fitting piston in a bore that is around 2" long. 2" is quite a bit of stroke. IMOA I suspect by the time the carrier moves 2" the bullet is long gone and there is no pressure remaning to do any pushing. in fact it is possable that at that point the port may become a restriction and the pistion will actually be pulling a vacume an actualy hurting things. it will certainly be slowing things down on the return cycle as it tries to push air back through the port. kind of like a tire pump. IM ONLY THEORISING HERE. but if your piston is sealed then every thing it takes in is going to have to be pushed back by the return stroke through the gas port. in your case the .169 port is over half the size of the bore so it it not likely much of a restiction. but in the case of my 223 with a .075 it would be if I were trying to pump 5" of air back through the port. HMMMM

As you stated the Impulse of the gas is all that is need to make it work as proven by the video of the AK firing with the gas tube removed. on a stock build the tube is nothing more than a guide. I will agree that sealing or reducing the clearance in the gas block and slightly beyond for the inital impulse may make a small improvement. as at that point any thing that goes past the piston is certanly not moving the piston.

I don't know if it will or even can but I'm trying. I also believe that IF the bullet (85g) were heavier, there'd be more intertia and stronger gas pressure. Just my thoughts
The bullet weight will have nothing to do with it. if the camber pressure is the same for a 85 gr or a 285 gr that is what is going to come out of the gas port. lighter bullets get more powder because there easier to move more powder is required to make a certain pressure. but your loads are usualy loaded to acheve aproximently the same pressure.
in other words if say your load makes 45,000 psi with a 185 and you swith to a 85 grain bullet and it makes the same 45,000psi then that is what the the port sees momentairly. there will be a slight variation in the amount of time there is pressures there as the heavier bullet will be in the barrel longer due to the slower velocity. but at the time when the bullet clears the port it will be the same.


The port is .169" located 7" from breech
that is a huge port over half the size of a 7.62 ak bore. (as large as the bore on my 17-223) it certainly must be hurting velocity as in a way it is making you barrel effectivly much shorter. it is bleading of a lot of what makes the bullet go forward. Hmm not sure if it is actualy bleading off as you say your pistion is very tight thus no escaping gases. HMMM again. I am concerned that you are porting that much pressure and volume into a sealed Gas tube. that tube is seeing a hell of a lot of pressure and volume of gas the same amount of pressure as the barrel at the point of the port.(what is the chamber pressure of the x25???) Id be carefull. if you were to ever stick a bullet in or obstrut the bore the likley hood of the carrier coming out the back of the gun is greatly increased with that much gas and pressure pushing on it and no way for it to vent.

another thing that comes to mind is how do you plan to maintain the seal with all the carbon ,and lead shavings crud ect that is imeaditly going to get into the the bore and piston?? with a port that big I would think there would likley be some shaving, surely it is going to wear very quickly. jamming is another thing that comes to mind. Im concrned about the wear on my set up and it vents after only aprox 2"

I am not trying to slam your build in any way. just expressing what to me are concerns or questions. if there is a way to run one of these with a smaller port Im all for trying it on the varmit gun.
 
#27 ·
Good discussion. I just ordered a 13" 7.62x25 barrel and will have a 4" tantal break on the end of it. I was thinking alot of this before I read it here. I noticed the exhaust ports on the gas tube/block intersection and realized most of the pressure is only maintained in the gas block so it would fire withougt the tube. The holes will vent the pressure once the piston has passed this position(the end of the gas block). I was thinking of closing these holes and moving them back some distance to allow for a longer pressure wave. I am possibly also going to use an FAL piston (if I can get it to attach to the carrier) and FAL tube with the reasoning that a smaller tube and piston will create a smaller pressure drop as the area expands when the piston moves rearward. The FAL tube actually fits inside the ak tube so I can retain the AK look. What do you think?

Edit: Was wondering if the gas tube is as strong as the gas block since I don't want a blow out. Guess we'll see. If so maybe I can weld an extension to the gas block.
 
#28 ·
I like the idea of a 13" barrel with the tantal break, I bet that will both look great and operate well. I'm curious to know where you bought a 13" tokarev barrel? As far as FAL piston/tube idea, don't forget how fast the bullet is going. As soon as the bullet clears the barrel the pressure will drop rapidly. You piston has only moved a fraction when this happens. Don't count on such a long pressure duration, pluging the gas ports should help but I think the FAL piston might be over-engineering the design.
 
#29 ·
Howdy Bullpup, I just keep reading how no one can get enough pressure to get the carrier to cycle properly without cutting springs and shaving metal off the carrier. I would like to avoid that if possible. It seems to me there is enough pressure in the barrel, it just needs to transmitted to the carrier better. Maybe port distance from the chamber and port size are the two big ones as well as barrel length. Maybe I should have kept the barrel 16" instead of 13". Should be a fun project to work on. I got the barrel from someone on the board here. He was rechambering several barrels and I bought one. (Cool)

Here is a video of the HK 416 piston driven rifle:
Comparing Carbines: Heckler & Koch 416 -- Army Times

Not sure if the video is "correct" in the timing of the bullet vs. the piston though.
 
#30 ·
Maybe port distance from the chamber and port size are the two big ones as well as barrel length. Maybe I should have kept the barrel 16" instead of 13".
You are right on those points, port size and port distance, carrier weight and spring strength are your major factors. Each is limited as to how much they can vary. The port can only be so large for the bore, Spring can only be shortened so far before it doesn't reach the carrier, you can only take so much weight off the carrier and still have it operate, the port can only be so close to the chamber.

I think you can get operation with a good balance of each factor, you have to determine what your priorities are. I opted for a build where the carrier was unaltered; so I put the gas block very close to the chamber; this has two benefits. The pressure is higher closer to the chamber, and the shorter 1" piston weighs less than a full length or AMD-65 length piston. Coupled with a slightly shortened spring and moderate port size, I expect to get pretty good operation. My receiver is in the mail, so I'll know soon enough.
 
#33 ·
I got my 7.62x25 barrel in and it looks great but it is going to take some work to get it down to size (it's fat) for the barrel parts to fit as I don't have a lathe. I need a kit for it also and at the moment I am going to build up my FAL while receivers are available. Will update when gas block placement begins.
 
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