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Thread: Will this blow me up? 7.62x25mm barrel

  1. #21
    Indian Admin Winn R's Avatar
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    Quote Originally Posted by Bullpup View Post
    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.
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  2. #22
    No Hope For Me 1biggun's Avatar
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    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.

  3. #23
    Gunco Member Bullpup's Avatar
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    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.

  4. #24
    No Hope For Me 1biggun's Avatar
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    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

  5. #25
    Gunco Member cntrailrider's Avatar
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    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

  6. #26
    No Hope For Me 1biggun's Avatar
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    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.

  7. #27
    Gunco Member Armadillo's Avatar
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    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.

  8. #28
    Gunco Member Bullpup's Avatar
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    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.

  9. #29
    Gunco Member Armadillo's Avatar
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    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.

  10. #30
    Gunco Member Bullpup's Avatar
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    Quote Originally Posted by Armadillo View Post
    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.

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