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

  1. #41
    Happy Camper hcpookie's Avatar
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    Quote Originally Posted by 1biggun
    I belive the veper uses a rpk trunion in it's 308 version and as far as I can tell the yugo m70 trunions and the rpk are the same. If some one know's of a differance I would like to know. I have seen some reloading sites that show the 7.62x39 loaded up to 50,000 CUP pressures with no prolems http://www.ak-47.net/ammo/reload2.html & http://www.chuckhawks.com/7-62soviet.htm.
    http://www.hodgdon.com/data/rifle/223rem.php In fact at over 51,000 cup the 223 is as high as a 308 or a 30-06. the m77 (308) and m76 (8mm) appear to use a m70 style trunion. a lot of this has been coverd in the 308 conversion forum that I started several monthes ago. Now I relize that chamber pressure and recoil are two different things and that a 8mm with a 200 grain bullet with only 39,000 cup is going to kick a hell of a lot harder than a 223. this is the force that works the reciver ,stock, rivits screws your shoulder ect. the chamber pressure is what works the bolt and trunion lugs. If I have this wrong let me know.. this is what let's me belive that a 308 or 8mm on a m70 stamped bulged trunion is safe. The milled version it thinner than the stamped m70 yet thicker than Yugo stamped.
    You're on the right track, and please don't get me wrong - I *DO* believe it can be done, given certain considerations...

    This is NOT something I'd buy a box of cheap '60s surplus Turkish tin-canned ammo and start blasting away. If it were MY face that was making a cheek weld onto that converted build, I would make darn sure that the ammo was a known factor - hand-loaded on the light side of the chart with lighter bullets.

    Personally I think .308 is the highest CUP of any of the calibers I've looked into. However we have a .308 conversion (Saiga) to use as a model. Honestly I can't think of any 8mm model other than the M76.

    To restate my conclusion, handloading "good" ammo makes this more expensive and therefore counter-productive IMO. I've got the bug for an 8mm MG42 only because that MG-grade "blaster" ammo is so darn cheap!

  2. #42
    No Hope For Me 1biggun's Avatar
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    Hcpookie we are on the same page. I would not trust a lot of that old 8mm ammo out there plus I hear that it is really dirty and corrosive. I am going to do the 308 first plan on shooting it with NATO loads for fun and hand loads for hunting. If I get the mag and feeding working I would probably build a 358-08 for a heavy pistol round http://www.hodgdon.com/data/rifle/358win.php. and a 243 for varmits. the only differance would be the barrel as they all use the 308 parent casing. 7mm-08 is another good choice as well. just bought 3 more yugo gits it beter work.

  3. #43
    Gunco Veteran SA58's Avatar
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    I think you need to excersise some caution running ANY surplus 8mm through ANY semi-auto (sans MG's)... but that's just common sense. I would think you'd be fine however, with S&B or some other commercial load. Especially that stuff I think Winchester puts out?? Heard it's loaded on the lower end of the spectrum.

    All that aside... sounds like the most critical area would be the bolt and probably the carrier, I believe the trunnions are the same.
    ...I'm not tense! Just terribly, terribly alert.

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    Happy Camper hcpookie's Avatar
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    See I haven't even looked at what's out there other than surplus.

    I think the bolt lugs are the most critical area, as well as the chamber area. Winn's toothpick test has proven that the trunion experiences less force than for example the rear stock tang/rear trunion area due to recoil forces. The bolt lug lockup is the area that I would be most concerned about. A properly headspaced gun shouldn't expose any area, but if you have a weak cartridge it is gonna blow out at the back. A ruptured cartridge in the chamber is no sweat - you get a case extractor and you're back in business. That "open" area is IMO the danger zone. Unless you have a low-quality barrel.

  5. #45
    Gunco Veteran SA58's Avatar
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    Well.. according to grasshopper, the 8mm bolt locks up in an M70 trunnion. Visually the lugs look the same (only seen the pictures, haven't truly mic'd it)... it just appears that the bolt face/head is longer than an M70's.. putting that hole for the extractor farther away from that lug. Knowing that 8mm's have been built (as well as .308's, and 54r's) I would think that if you had an 8mm bolt and carrier (for good measure) with an M70 trunnion.. you'd be good to go, safety wise. At least as safe as any other 8mm.

    Ya... the winchester 8mm hunting ammo is loaded very mild from what I understand. The S&B is loaded more to European specs with a 196gn bullet. I guess it would help to know what the Yugo's shot through their M76's... but I suspect it was the same stuff they ran through their M48A bolt guns.
    ...I'm not tense! Just terribly, terribly alert.

  6. #46
    Gunco Veteran SA58's Avatar
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    Quote Originally Posted by hcpookie
    See I haven't even looked at what's out there other than surplus.

    I think the bolt lugs are the most critical area, as well as the chamber area. Winn's toothpick test has proven that the trunion experiences less force than for example the rear stock tang/rear trunion area due to recoil forces. The bolt lug lockup is the area that I would be most concerned about. A properly headspaced gun shouldn't expose any area, but if you have a weak cartridge it is gonna blow out at the back. A ruptured cartridge in the chamber is no sweat - you get a case extractor and you're back in business. That "open" area is IMO the danger zone. Unless you have a low-quality barrel.
    I'll have to take a look at my Hakim and see how much cartridge hangs out of the chamber... seems to me I remember thinking it seemed like a bit much, but there you have a professionally engineered 8mm semi. Apples and oranges when compared to an AK, I know.

    I picked up some MG-13 mags and it looks as though they could work after you mod a front and back lug. The mag itself is thinner than a standard AK mag, wich would give you some room to work with said mods. (anyone know where I can get junk AK mag bodies to cannabalize for dirt cheap??) I have a fixed stock Yugo kit to work with on it's way, and I have a possible line on an 8mm bolt and carrier... that part is key. I'm sure a Saiga .308 could be made to work... but do to the fact you'd have to chop a rifle up, the trunnion is sorta Saiga specific in that you'd have to use that Saiga reciever.. wich would mean a whole lot more work as far as moving everything back to accomodate that mag. Ah well... we'll see. If nothing else, I'll have some extra parts to peddle if it doesn't look like it'll fly.
    ...I'm not tense! Just terribly, terribly alert.

  7. #47
    Gunco Regular Shadow Walker's Avatar
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    My plans are: A M70 Yugo parts kit, a Coldsteel Solutions blank M70 receiver, and modified MG-13 magazines.

    Push the FCG back about 1/4 inch, and the crosspin moved back as well to clear the MG-13 magazine. The hammer may have to be grounded down to clear the crosspin.

    I already have everything, I just need a new shop so I can start building.

    I gave away the three cases of hot 8mm ammo I had, so I should be O.K.

  8. #48
    Gunco Veteran SA58's Avatar
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    I was also looking at those Romak III PSL blanks by Coldsteel... I was thinking that it should have the correct length with nothing getting in the way, and they've got a nice long mag dimple. I have no idea how different the bulges are from M70 to Romak though.

    Just an FYI...
    I took the micrometer to the barrel of a 8mm Yugo M48A and one of my other Yugo M70 kits.... the barrel diameter, at least towards the muzzle.. is identical. Very cool... means I should be able to utilize the gas block and front site from an M70 kit as well as the RSB.

    Anybody get a good look at that grenade cut-off switch?? Anyway that can be cleanly modified into an adjustable gas block??
    ...I'm not tense! Just terribly, terribly alert.

  9. #49
    Happy Camper hcpookie's Avatar
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    Quote Originally Posted by SA58
    Anybody get a good look at that grenade cut-off switch?? Anyway that can be cleanly modified into an adjustable gas block??
    I think or Winn did that... definitely looks doable.

  10. #50
    Gunco Rookie TXaggie's Avatar
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    Default CUP and PSI

    There seems to be a bit of confusion on CUP and psi around here. Psi is a more modern measure of pressure. CUP and LUP were the standard for many years, and you will find references to it in handloading books for some time to come. CUP and LUP are copper and lead crusher pressures respectively and are actually the same measure of pressure, they are interchangeable. Psi and CUP/LUP are not interchangeable, nor is there a conversion factor between the two. CUP is a caliber specific measurement. The same CUP in one caliber is not the same as the same CUP in another caliber. 52,000 CUP in a 6mm Rem is not the same as 52,000 CUP in a 223. The psi difference between the two is 10,000psi. I know this makes no sense so just check out the attached table. It came from a Hercules handloading pamphlet, it?s not BS.
    You can?t really say how much pressure an action can take, you need to calculate bolt thrust. The amount of reward thrust placed on the locking system is what determines what an action can take. When fired, a case pushes backward in response to pressure like a hydraulic piston. The total amount of reward thrust is determined by pressure and area for the cartridge base. Would you rechamber a crappy old 1 MK3 Enfield and rechamber it to 45-70, 45 ACP, 7.63x39 or even .223 Rem? Probably not, but lets do some quick math. To calculate bolt thrust you need to know the max cartridge pressure in PSI not CUP/LUP, and the cartridge case base diameter, which is the diameter of the case above the rim or extractor groove, not the head or rim diameter. You can find both in a good loading manual such as Speer or on line at Ammo Guide. Using the case base diameter, find the area of the case in cross section, which is what you see if you stare down the neck of a fired case. The formula is (radius) squared * 3.14 or you can use (diameter) squared *.785, either will get you the roughly the same answer. Once you get the area multiply it by the max pressure in psi not CUP. This is the max amount of rearward thrust placed on the locking system in pounds. Here are the case dia and max pressures of the cartridges I mentioned above.
    Cartridge Case base dia Max pressure Rearward thrust lbs.
    303 British .455 49,000 7963
    45-70 .470 28,000 4855
    9mm para .391 35,000* 4200
    45 ACP .476 22,000* 3913
    223 Rem .375 55,000 6071
    7.62x39 .438 45,000* 6777
    * pulled these off the web, couldn?t find max in psi in my books, only CUP.

    Take a good look at the cross section area and max pressure columns, there is a relationship between them and bolt thrust. If the pressure and area are higher then the total thrust is higher, reduce either and the thrust is lower. The 223 has the highest pressure but not the highest thrust. The 45-70 has the highest area but not the highest thrust. Note that the 45ACP and 45-70, have similar head diameters and pressures, thus they produce similar (sort of) rearward thrust. Also note that the 7.62x39 actually produces a higher thrust than the 223. Of interest to those doing pistol caliber conversions is that the 9mm actually puts more stress on the locking system than the 45ACP, but in a blow back system that?s a whole nother can of worms. By the way, As far as I know the 1 MK3 Enfield has been reliably chambered in all of the above cartridges except 223(case head is too small for the bolt face).

    I also worked out the bolt thrust on the following cartriges for reference.
    Cartrige Case base dia Max press psi Bolt thrust in lbs
    7.62x39 .438 45,000* 6777
    8mm Mauser .470 35,000*Saami 6069
    30-06 .470 60,000 10404
    308 .470 60,000 10404

    If you think I?m full of it (I would be doubting this too) go over to the Roderus Custom forums and look in one of the 50BMG construction threads (don?t remember which one) and check out how to do the numbers yourself. I don?t want to come across in the wrong way, I?m not trying to piss on anybody?s parade, I?m just trying to share what I have learned from others, so that we can be safe, I like my face the way it is. The point I am trying to make is that max psi in a cartridge does not determine if it is safe to shoot in a specific action, bolt thrust does, and that psi and CUP are not the same thing. If you still believe that pressure is the sole determining factor of what and action can take, consider this. The max pressure of a 22lr is 24,000psi while a 12ga is only 11,500psi; a 12ga action is a hell of a lot stronger than most 22lrs.
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