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6.8 vs 7.62x39

Discussion in 'Rifle Country' started by TEX, Jul 27, 2007.

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  1. Quiet

    Quiet Member

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    Just messing with you. :p

    I concur with your assessment that it's .277.

    .270 is just what the manufacturers calls it. Probably for marketing reasons. Kinda like why .38 Special & .357 Magnum are really .355 and .50AE is really .495.
     
  2. alucard0822

    alucard0822 Member

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    SSK also developed a round to suplement the 5.56 that was given a trial along with the 6.8SPC, it was the 6.5MPC. The MPC had a 107gr bullet that would achieve 2400fps out of a 12" barell, and around 2600 out of an 18", it had the same OAL, rim and width of the 5.56, but was basically necked up a little, and some minor tweaks to fit more powder in the case, but basically the only thing that needed to be changed on M-16 or M-249 platforms were the barells, even the standard 5.56 mags and links worked, too bad that one didn't get the green light, brass, bullets and a barell or upper would be all I need. 6.8 seems neat, but unless cheap surplus is expected (Im not holding my breath) I am going to stay wih my AK and AR rifles. For my purposes there is not much 6.8 will do that either of these can't.
     
  3. GunTech

    GunTech Member

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    6.8SPC uses bullets that are 0.277. Due to magazine length, you are limited to bullets under about 115 grains.
     
  4. HorseSoldier

    HorseSoldier Member

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    A 6.8mm Rem SPC version of the SCAR-L was in the cards until SOCOM's trials of the round indicated they'd prefer to stick with 5.56mm. The change would still be about as easy as changing caliber on a LMT MRP upper -- barrel and bolt and mags and you're in business.
     
  5. SpeedAKL

    SpeedAKL Member

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    6.8 supposedly gives you most of the 7.62x39's knockdown power at close range while shooting even farther and flatter than the 5.56 NATO. One thing to consider is price, though - good luck if you can find 20-round boxes of 6.8SPC for under $18-20...
     
  6. Coronach

    Coronach Moderator Emeritus

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    The SCAR's development in multiple calibers is, as I understand it, based upon two frames, the SCAR-L and SCAR-H. The "light" version uses a 5.56x45 M16 magazine compatable well and the heavy uses a 7.62x51 magwell of some sort (M14? Proprietary? I'm not sure).

    Unless my statement about the SCAR-L being based upon 5.56 NATO dimensions is incorrect, I think the basic gist of my comment remains true. It's not like they allowed for the development of a 6.8-esq cartridge that is 46mm in OAL for the SCAR-L. It's still tied to the OAL if the 5.56 NATO cartridge.

    Mike
     
  7. Zak Smith

    Zak Smith Moderator Emeritus

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    I thought 7.62x39 played in there somewhere, considering SPR-V. Online references say the SCAR-H was convertible to 7.62x39 (??).

    The only other thing I have to add is that being designed around the M16 magwell vs. "designed around 5.56 NATO" need not be the same thing. The viable assault rifle cartridges are all more or less the same length, and I think most would agree the action and weapon size resulting from 5.56/7.62x39/etc-sized cartridges is compelling over longer cartridges. There's no need for the mag-well to be fixed in one of these modular weapons systems-- viz, the Masada.
     
  8. Coronach

    Coronach Moderator Emeritus

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    Right, but what I'm saying is that the SCAR is designed that way. That, combined with the fact that 6.8SPC was mandated to fit within standard M16 magazine dimensions, is evidence that they're not seriously thinking about moving too far beyond what we have right now. If you were thinking about doing something different, you would design a new weapon and a new round without the limitations of either of the old components.

    I agree it does not have to be that way. They are, however, doing it that way.

    Mike
     
  9. GunTech

    GunTech Member

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    Agreed. What the 6.5/6.8 should have been.

    Personally, I'll take 6.5 as there are better bullets, so long as we are not confined to M16 magazines.

    The ideal round should be relatively light vs.the 308, moderate body taper for good feeding, have goot short range killing power, and good moderate range petformance. Ideally, it shouyld also be effective at long range for DMRs and GPMGs. It should also have relatively low recoil for full auto use.

    For my bullet I pick the 6.5 123gn Lapua Scenar with a BC of 0.547. I want the recoil under full auto to be controllable, so it must not be much more than the AK. We set our velocity target from an 18inch bbl for 2600fps. With a rifle weigh of about 9 pounds stripped, this gives us more recoil than the M16, an a touch more than the AK. Assuming a muzzle brake, this is reasonable recoil, even under full auto. The 6.5 grendel can manage 2400 with this bullet, so we will require a longer case to gain velocity and enough taper to ensure reliability.

    How does the round compare with the current issue 5.56x45 and 7.62x51?

    Recoil: The 6.5 has about half the recoil of the 7.62x41, and about 1.8x that of the 5.56.

    Energy:

    Code:
    cartridge	0	100	200	300	400	500	600	700	800	
    M855		1239	1019	832	673	538	426	335	264	210
    6.5		1846	1649	1469	1305	1155	1019	896	787	689
    M80		2502	2145	1829	1551	1306	1094	912	758	631
    
    the 6.5 proposed comes very close to 308 at moderate ranges, and overtakes it long range. This with a lighter bullet going slower

    A second variation of the 6.5 using the 108gn Laupa scenar at 2800 fps yields slightly less recoil, about 1.5x that of the 223.

    Code:
    cartridge	0	100	200	300	400	500	600	700	800	
    M855		1239	1019	832	673	538	426	335	264	210
    6.5		1880	1658	1458	1277	1114	968	838	722	621
    M80		2502	2145	1829	1551	1306	1094	912	758	631
    
    It seems that we can pretty easily create a cartridge that comes very close to 7.62x51 without the heavy weight or recoil of 308. Get the bullet to fragment like the M855 and M193, and it beats the 5.56 in every way.
     
  10. carnaby

    carnaby Member

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    The bullets called for in all the loads are the same as used in the .270 Win. I've used 110 gn, 115 gn, and even 130 gn bullets in my 6.8 loads out of my AR. I suppose the 130 grain loads come out pretty slow, but they will certainly be affective on game out to 200 yards. Shooting at 200 yards, I notice about 8-10 inches more drop than the 115 grain bullets, and the load data guesses that they come out around 2200 fps compared with 2600 fps for the 115 grain loads. I don't know for sure, since I don't have a tach.
     
  11. GunTech

    GunTech Member

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    You can get a pretty good estimate of velocity without a chronograph iof you measure drop and get the barometric pressure where you are shooting.

    What rifle are you using? I thought about 130s, but didn't like a compressed charge to seat them deep enough in the mag. 110s and 115 seem like the best balance, but Zak is the 6.8 guru.
     
  12. carnaby

    carnaby Member

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    I'm using a home-built 6.8spc with a barrel from ko-tonics. Has an "improved" chamber that can handle higher pressures. Pretty nice rifle.
     
  13. GunTech

    GunTech Member

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    OK, 6.8 in an AR-15.

    Some are using 6.8 in a turn bolt, which give a little more room for longer bullets.
     
  14. HorseSoldier

    HorseSoldier Member

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    I believe you are correct. From what I understand, the 7.62x39 versions or conversions are going to be a low basis of issue kind of niche weapon to replace the SR-47s.
     
  15. Grendelizer

    Grendelizer Member

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    Just a couple corrections:

    GunTech wrote: "Such an extreme taper may not be necessary for high reliability, but the essentially non-existent taper of the Grendel is a liability in a weapon that may be exposed to extreme conditions."

    Tod, the Grendel has more taper than 5.56, more taper than 6.8, more taper than 7.62 NATO. Only 7.62x39 has more.

    GunTech wrote: "They keep saying 6.5 is coming from Wolf, but I have yet to see it."

    It's here, and it's been here. I just shot 80 rounds of it yesterday! Alexander Arms, of course, sells it. Cabelas sells it. MidwayUSA sells it.

    John
     
  16. GunTech

    GunTech Member

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    Grendelaizer, can you provide a cite?

    According to my references (SAAMI and CIP), here's how the cases stack up. Note that 6.5 grenel is neither SAAMI or CIP.

    Body taper

    7.62x39 -- 1 degree 29 minutes 5 seconds
    5.56x45 -- 30 minutes 5 seconds
    6.8 SPC -- 29 minutes 3 seconds
    6.5 Grendel -- 22 minutes 23 seconds.

    Thus 6.5 grendel has the least body taper of the four catridges listed, not suprising since it is based on the 6mm PPC, a blown out 7.62x39 with a sharp shoulder.

    6.8 and 5.56 have nearly identical body taper, and the 7.62x39 has aroun 3x the taper of the 5.56.

    BTW, 6.5 Grendel is better than the 308, which has a body taper of 20 minutes 40 seconds.
     
    Last edited: Jul 31, 2007
  17. Grendelizer

    Grendelizer Member

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    Tod, it looks like you've done the math and I was just repeating something I'd heard.

    Please tell me how to do the math, and I'll measure a case when I get home tonight.

    John
     
  18. SaMx

    SaMx Member

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  19. woof

    woof Member

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    Is the Ruger mini in 6.8 on the market? Anyone here have one?
     
  20. aspade

    aspade Member

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    It's just trig.

    Using .308 as an example:

    Take the diameter at the head where the extraction groove bevel ends and at the shoulder - 0.470" and 0.454" respectively. Take the distance parallel to the bore between those two points - 1.435".

    This gives you an isosceles trapezoid. Imagine trimming off the edges to leave the largest possible rectangle. The trimmings will be two right trangles with dimensions 0.008" perpendicular to the bore. and 1.435" parallel to the bore.

    Let the side perpendicular to the bore be a and the side parallel to the bore be b

    tan A = b/a. Which gives 0.314 degrees. Multiply that by 60 to get minutes - 18.84 minutes, or 18 minutes and (.84*60) 50 seconds.

    This calculation does not exactly match the 0'20'40 spec, due to rounding errors or not measuring the same points on the case, but it's close enough for internet debate.

    Back to the original topic of feeding, shoulder angle is at least as important as body taper.

    6.5 Grendel - 30 degrees
    .223 - 23 degrees
    6.8 SPC - 23 degrees
    .308 - 20 degrees
    7.62x39 - 18 degrees
     
  21. GunTech

    GunTech Member

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    I cheat and derive the body taper in QuickLoad using either the SAAMI specs or CIP. Good point about shoulder angle too. IU meant to mentio it.

    There's another thread about the ultimate combat round that this discussion dovetails nicely into. Think about what round you'd spec out if you weren't constrained by M16 magazine size.

    Right now, I am thinking about something that

    1. Has no more recoil than an AK in 7.52x39
    2. Has superior energy at all ranges than the 223
    3. Is suitable for a GPMG or DMR, with enough retained energy to be effective at 600+ meters
    4. Weighs less than current full power cartridges
    5. has pressure levels suitable for light weapons

    I think the 6.5 is very close, but not quite there. I'm going to pick an arbitrary case head, and select the 30 remington/6.8 with it's 0.422 case head. I opt for a 1 degree body taper (halfway between 223 and AK) and a shoulder taper of 20 degrees. As a starting point, I pick the 6.5 using the Lapua Scenar 108 gn with a BC of 487. My target velocity is about 2800 fps in an 18 inch bbl.

    More data to follow. I need to spec out my round in QuickDesign.
     
  22. MudPuppy

    MudPuppy Member

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    Not to stray too far off, but I could have sworn that tony rumore (tromix) did an AK conversion into 6.8? (and 458 as well?)

    I'm pretty sure some of the AK homebuilders on gunco have done 6.8s.
     
  23. GunTech

    GunTech Member

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    Interesting. I would be touch to do the bolt face conversion on an AK, I would think. I suppose you could open up the bolt face on a 223.

    I did a 6.8 conversion an a Keltec SU-16 and that was a major PITA. You can see the details on ktog.org
     
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