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Thread: 308 ak

  1. #121
    rbthntr64's Avatar
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    THe Vepr has two lugs.
    Vepr II .308 is a Keeper!
    Freedom is not free. Some will perish to preserve it for the many. Just as our Forefathers did before us, we must take up the battle and not waver. Victory is our only option.

  2. #122
    Gunco Regular Gunter's Avatar
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    Today I did some measuring between a 7.62 AMD, 7.62 Romy, and a 5.45 Bulg. bolt lugs. The AMD and Romy were within .001 of each other so we will just call them the same. The 5.45 has the smaller stem then the 7.62. What I'll call the small lug is the one away from the extractor. The width and height are the same between them on either caliber. The difference is in the length. The AMD/Romy is .402 long and the 5.45 is .551 long. The large lug where the extractor is located is .611 long on the AMD/Romy and .720 long on the 5.45. The bolt face hole on the AMD/Romy is .458 and the 5.45 is .400. Both would need to be opened to around .475 for the 308.

    Rbthntr64, does that sound like the measurements on the Vepr? Also didn't the Saiga 308 use a small stem bolt?

  3. #123
    Gunco Regular Gunter's Avatar
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    Math isn't my greatest subject either so, someone check these figures.

    In the other thread 1biggun asked about bolt thrust at 42,000 cup on a .308 and 50,000 cup on a 7.62x39. I found a formula to convert cup to psi. Now this will only be a ballpark figure as there are +/- in both cup and psi. So hitting either end of the +/- could give different results. The formula is Cup x 1.516 -17902 = PSI. So, taking your .308 at 42,000 cup should be around 45,770 psi. The 7.62 at 50,000 cup should be around 57,898 psi. Now we need to figure the square inch we have touching the bolt head. The .308 is .473 inch across so, the radius is .2365. We take .2365x.2365x3.14=.175 square inch of area (I think). The 7.62 is .444 inch across so the radius is .222. So .222x.222x3.14= .154 square inch of area. Now we can take the psi x the area to find the amount of pressure on the bolt. For the .308 at 42,000cup/45,7700psi the force on the bolt is 8009 pounds. The 7.62 at 50,000/57,898psi the force is 8916 pounds. If we bumped the .308 up to 60,000 psi the force is 10,500 pounds. Now none of these figures take in to account the friction between the brass case and steel camber. These are just pressure figures at a certain pressure on a certain area.

  4. #124
    No Hope For Me 1biggun's Avatar
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    now we are getting some were. great work. I have not seen the formula for converting C.U.P to PSI every thing I had read said there was no direct comparison must be why the + - disclaimer

    just for a direct comparison what differance does the bolt head make in loads of the same pressure. lets use the hottest safe load in the current Lyman book for the 7.62 x39 of 44,700CUP of rx7 powder moving a 123 grain bullet at 23,000 fps versus the 308 win with 44,700 CUP with the same bullet. (apples to apples)

    308 load is suprisingly aprox 700 fps faster. the 7.62 was shot from a 20" barrel the 308 was from a 24" these are bolth puplished loads. the 308 has loads starting at aroud 29,000 cup to 52,100 for this bullet. sorry for listing every thing in C.U.P but all the 7.62x39 loads I have in writting by a repitubal sourace are C.U.P specs. the 308 has loads listed bolth ways but not for each load.

  5. #125
    Gunco Regular pupwag's Avatar
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    Saiga 308 uses the small stem.Extractor side lug is the same as Bulgy 74 ~.720.The ejector side lug is .100 longer on the 308-.645.Most of that is a ramp to help rotate the bolt.Bolt face hole looks like .478.Bottom part that strips the rounds is the same as a 47 width/depth wise, of course the Saiga has the little (third)sorta lug projecting down off the rear left side .The Saiga bolt was ~.050 wider across the two main lugs with little reliefs cut into the lugs to clear the upper rails as it would have been to wide to fit the ~.980 width opening for the bolt carrier.It would not enter a regular AK trunnion until I milled off the .025 per side in a rotary table.
    It's only metal, it's not magic.

  6. #126
    No Hope For Me 1biggun's Avatar
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    I am going to back of my statment of known safe loads by saying thy are LYMAN puplished loads. I have know idea if they are safe in a AK47. they may only suitable in a bolt action there is no mention as to what action sks, AK or bolt action they were shot in. I suspect they are ok but I just dont want to miss lead any one if I dont have cold hard facts.

  7. #127
    Gunco Member bert01's Avatar
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    im going to talk with a couple of the engineers tomorrow at work and get them to do some calculations with the trunnions and bolts and see if they can come up with some shear numbers for the parts. ill post what they come up with asap, they do this stuff every day for industrial equipment, and i would put a lot of faith in whatever they come up with

    then we can compare those numbers to the bolt thrust numbers we've come up with

    has anyone tried to find out what kind of numbers the "strict european pressure tests" used in deciding the romy 308's couldnt be sold to the public

  8. #128
    Gunco Regular Gunter's Avatar
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    Here's a link to the pdf article on the cup/psi conversion if you want to take a look at it.

    www.shootingsoftware.com/ftp/psicuparticle2.pdf

    I'm not an engineer, don't play one on tv, and didn't stay at a Holiday Inn Express last night, so take the above info with a grain of salt. Bert01's engineers may say that everything is BS. I know that there is software out there that the engineers can use to test stress and how a metal will hold up over a certain number of duty cycles. Maybe we can get some help in sorting all this out.

    Looking at the different calibers:

    The 5.45 pushes a 60 grain bullet at around 2900fps. Surface area is around .120 square inch.

    The 7.62 pushes a 123 grain at around 2350fps.

    The .308 pushes a 147 grain at around 2800fps.

    These are the figures for the 7.62 and .308 are from PCM, UMC, Winchester white box, and American Eagle as these are probably the ammo that someone would just walk in and buy at the store. If we could find the pressures that these rounds are running then that might give us a chance to compare apples to apples. Does anyone know the data on the Yugo .308 ammo?

  9. #129
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    Here's a link about the CIP and SAAMI standards for 9mm. It goes into some detail about the two. The CIP lists their loads as MPa (which 1 MPa = 145.0377psi), but the post states that you should not convert the two.

  10. #130
    No Hope For Me 1biggun's Avatar
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    some good info here on SAAMI specs http://www.frfrogspad.com/miscellk.htm shows the max PSI for the 7.62 at only 45,000 psi?? US made weapons would have a max proof done at 63,000 psi. I am suprised that my Lyman book shows MAX loads of 44,700 C.U.P which would be 50,318 PSI +- according to the above formula. makes me wonder? I am sure there are people out there using these loads in there AK,s what is reasuring to me is there is a load listed using WIN 748 powder listed as a max load only making 43,200 C.U.P and pushing a 150 grain bullet at 2996 fps this is a huge increase over the 7.62x39 in bolth performance and and knock down power a perfect deer hunting round. this converts to 47,589 PSI and only about 2,589 PSI over the SAAMI spec for a store bought sold in the USA AK 47 ! dont all guns sold as US made have to meet the SAAMI specs?? I think this load would possably be ok even on a ROMMY build. I am thinking more and more about turning my case heads down to fit the bolt and doing a quick Rommy 308 build. turning the head would prevent accidental loading of a 60,000 PSI load. which by the way would still be under what the gun would be proof tested at in the USA.

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