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Thread: 8mm AK... how hard could it be??

  1. #51
    No Hope For Me 1biggun's Avatar
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    Not disagreing with you (still thinking) so what you are saying is that a8mm mauser(6069 thrust) puts less thrust on the the lugs than a 7.62x39 (6777 thrust)? you show a 308 and a 30-06 the same even though one has a larger case capicty for example the30-06 uses 4 grains more powder (vargert) than a 308 with the same bullet. at the same cup pressures. If the above calculations were true a 22-308, 308, 338-08, 458 win (same parrent case) loaded with different bullet weights to the same Cup pressures would all produce the same bolt thrust? you havent rained on any ones parade in fact just the opposite. It means that one could load the 8mm up some. I'm still taking this in and I do not doubt the math .good info. The $64,000 question still is what is a safe thrust pressure for a AK (Yugo) bolt and trunion. My caculator is on fire right now thinking of a boltface as a HYD piston does make sense. the 458 SOCM with 50,000 CUP must produce huge thrust numbers ( just saw a pic that was posted on the CAL that may work forum today of a AK chamberd for 458 SOCOM)

  2. #52
    Gunco Rookie TXaggie's Avatar
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    The numbers I posted should be good for a manually operated action, I don?t know what other stress are imposed on a semi auto action. If you go over to the Roderus forums and look in the Ask a Guru section and look at the ?other 50cal designs? post (I think that?s the name) you will see a good bit of info on the stresses placed on the locks of a 50cal action. The difference is that sense these are home builds, they know the locking lug area and the material yield strength. If you can find out how strong the steel in an AK action is you could probably do the same. Actually, If you post a question in that forum you would probably get some good responses from knowledgeable people. I have never designed or built a rifle, I was just trying to explain the differences between CUP and psi. The info on the enfield I included was just an example because I have seen it chambered in those calibers.

    I think one of the difficulties you would encounter are the fact that the 7.62x39 bolt and carrier are going to be a bit short for the long 8mm cartridge. They may not be long enough to hold the rounds down in the mag. That may be the reason that the 8mm bolt has longer lugs, or it could be that the longer lugs are stronger, which they would be. It would be nice if someone who has an 8mm bolt and carrier would take a pic with ? grid graph paper as a background so you could get the basic dims, and also measure from the back of the carrier to the front of the bolt, that would give you the relationship to the position as it sasses over the cartridge in the mag. The pressure I posted for the 8mm was a saami spec, It is noted in my Speer loading manual as being low so as not to cause problems with the old small ring mausers. The surplus you get from Europe is no doubt loaded to a different spec. The military stuff may be quite a bit hotter than US specs. Arsenals often load ammo for specific weapons platforms for military use, so who knows what the pressure is.

    Another problem is where to put the gas block. It needs to be the right distance from the chamber in order to catch the pressure curve in the right spot, so that it has the right amount of gas to operate the system. You might want to ask the guys doing the pistol conversions with ultra short barrels there thoughts on this. The 1.5mm receiver on the Yougo is defiantly the way to go. Global had at one time mentioned using their 1.6mm receivers for ?larger cartridges?, its not on their web sight now, I guess it didn?t pan out. If someone here gets a 30-06 Siaga and would be willing to post pics and take some measurements that would help you a lot too.
    The $the 64.000 question I cant answer, but I think the Yougoslavian engineers already did, they redesigned the bolt and carrier. Technicly you should use the case ID to calculate thrust, but that info is hard to find, and besides using the OD will make the thrust come out slightly higher than it really is and will give you a very slight saftey margin. And yes please check my math, I take everything I find on the net with a grain of salt. I see a lot of stuff that just doesn't smell right.

    Sorry I am a bit scatterbrianed right now, but to answer your question about why two different cartriges use different amount of powder and prodece the same pressure, it depends on a lot of things. The throat length and style, bore dia, and bullet seating depth can all play a part in pressure differences. Longer throats and freebore tend to reduce pressure, and thaey are specific to the rifle not the cartrige. It depends on what the ammo manufacturers used for testing. The Speer #11 data for the 7.62x39 was taken using a mini30 which i have been told has a long throat to reduce the pressure of firing a .311 bullet through a .308 bore, yet Speer used .308 bullets in thier testing. There are alot of people shooting mil spec .311 in thier mini30s without any problems. Fire a .308 bullet in a .311 AK bore and im sure you would get a different pressure measurement. Also another reson for the powder and pressure differences in callibers is that some case shapes are more efficient at burning powder than others. If you have a load book handy check out the amount of powder required to push the same bullet to the same velocities in different cartriges. The bench rest and PPC series are very efficient users of powder, while the 30-30 and other less bottle necked designes are not. Pressure is really the result of a lot of things acting together and you have to take that into account.
    Just noticed something else
    "22-308, 308, 338-08, 458 win (same parrent case)"
    The 458 Win Mag is not based on the 308 case, it is a belted magnum and it has a base dia of .511.

  3. #53
    No Hope For Me 1biggun's Avatar
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    Yea I meant 358 not 458 also you can add 7mm-08 to that group. as far as I have been able to find the 8mm and 308 Yugo built m77 and M76 have the same size lugs and trunion. the bolt its self is longer. I have noticed a differance in the length of the lhgs between a milled and a stamped trunion/stub. If a AK trunion holds up to this it should handel a 8mm and 308 http://www.tromix.com/Tromix_458.htm> I plan to contact this guy to ask about barrel avalibilty for this round (why I had 458 on the brain). the gas blok is not a real issue the pics I have seen of the m76 looks like it is in a normal location. pistols seem to work about any were you put them. If you look at the 308 conversion forum there has been a lot of discusion and pics of bolt s trunions ect. If you haven built a AK I highly recomend it two years ago I could have carred less about one. now I do most of my hunting with a AMD 65 pistol scoped and all my friends now want one. I have acess to a 40,000psi hyd press with a guage on it that reads press pressure not hyd psi. I have considerd using it to press a locked in bolt until it failed. I relize that it would not be a true indicator of what is safe but it would give you a one cycle failure bolt thrust reading. If 308 win is at around 10,000 psi thrust I think I would like to see at least 15,000 psi in a test hold. I would need a good trunion and bolt to waste around 45$. again thanks for the Info I always felt I was missing something by using just chamber pressures.

  4. #54
    Gunco Veteran SA58's Avatar
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    ...all that info just gave me brain bubbles, good stuff that it is.

    As far as the location of the gas block, I imagine you could look at the placement of the saiga .308 and/or a Romy PSL.... it'd have to be similiar enough to give you an idea. Wouldn't the diameter of the gas port have an impact on this as well? As in, couldn't you tweak the location with a bigger or smaller hole diameter?? If the front gas block on an M70 can be made into an adjustable, you could pretty much make an oversized hole and fine tune it to function no matter where it was placed, I would think? ....and just out of curiosity, because I don't know any better... does the gas port need to be situated in a groove vs a land? and if so, how the heck do you locate the correct spot to do that??

    Bolt and carrier. I think that's the biggest issue with the conversion. How do you safely lengthen the bolt stem on an M70 (and do you want to).. and it seems like the carrier has a longer back end (on the M76) so how would you deal with that issue. Too bad it's not an easy task to locate an M76 bolt and carrier.
    ...I'm not tense! Just terribly, terribly alert.

  5. #55
    Gunco Rookie TXaggie's Avatar
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    Good spot on the 458 socom! If you run the numbers on the bolt thrust, that should answer your questions about how much an AK can take. Like you said a hydraulic press can simulate the total bolt thrust, but not the shock load of actually firing a round. I think testing a trunion and bolt in that fashion would only yield ruined parts and not much useful knowledge.
    Tromix is using a milled receiver and the general consensus seems to be that they are not any stronger than a stamped one, but my questions are why is he using one when they cost five times more than a stamped receiver?, and is he using a standard bolt or a siaga three lug? If he is using a milled receiver for some other reason than locking strength, maybe it?s just stiffer?, then I think your project is doable. I was pretty sure the gas block placement would not be that big of an issue, but just wanted some input to confirm, it seems to affect ARs more than AKs. Once you address the safety issue your biggest hurdle will probably be making the short 7.62x39 bolt/carrier work with the longer 8mm round. Lengthening the carrier would be impractical. You would probably have to use a PSL carrier instead. The bolt would not be that hard to do if you had access to a lathe. The rear of the bolt just behind the lightening groove could be removed and turned to a shoulder to accept a piece of 4130 tubing, that was hardened on the tip and soldered into place, to act as an extension. A PSL firing pin could be used, or a new longer spring loaded pin could be made. I?m assuming you are planning to bend your own blank that is longer than a standard AK, more like the PSL or ROMAK length, and that the overall length of the action won?t be a big issue. If so then fitting a MG13 or Madsen mag shouldn?t be too hard given the longer available length of the mag well opening.

    Here is some good info on bolt lug strength

    http://www.riflebarrels.com/articles...g_strength.htm

  6. #56
    No Hope For Me 1biggun's Avatar
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    The response I got back from Tony at Tromix was that he used a 7.62x39 bolt and a parts kit that he could not remember the origan of. there has only been two 458 SOCOMS built by him. My understanding is the one shown is his personal hunting gun. I also was wondering why he used a milled reciver also. I suspect it was to handel the recoil and to up the overall quality of the weapon. I think the saiga in 7.62x39 is only a two lug bolt (please correct me if I am wrong). I am looking into a reamer for this another site speaks of bolt actions being done in this round as well so the stuff must be out there. This is the exact response I got from Tony at Tromix.

    I have only built 2ea AK's in .458 SOCOM and both were on milled Firing Line receivers.
    I don't recall the extact parts kits that I used, but I know they were 7.62x39. The first gun was set up to run on modified bulgarian 5.56 waffle mags. The second gun (shown on the website) was set up to run both Galil and modified bulgarian mags. As for pricing, every customer wants some "special" crazy shit on their gun, so I have to price each one after I know the spec. The gun shown would run $1,650.

    I have no plans to sell barrels only.

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