New cartridge ?

Discussion in 'Videos Of Tech Stuff And Reviews' started by J E Custom, Feb 14, 2019.


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  1. J E Custom

    J E Custom Well-Known Member

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    Thanks
    More good information. The only part of the article I would have to disagree with is the part about the 6.8 SPC only being marginally better than the 556. It is almost twice as powerful and exceeds the 762x39 in every way except in bullet weight.

    I don't have a problem with them trying to improve the 6.8 but excessively high pressures are not the way. this will lead to failure at the wrong time for the solders using it.

    J E CUSTOM
     
  2. entoptics

    entoptics Well-Known Member

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    Can't deny the 6.8SPC is better than the 762x39, but I'm not sure 6.8 is anywhere near "twice as powerful" as 556. Just googled some load data, and 556 is in the 1100 ftlb neighborhood, while 6.8 SPC is around 1600 ftlbs (16" barrels). That's substantial, but not double.

    The ~115 gr class of 0.277 bullets doesn't even really beat the ~69 gr class of 0.224 bullets in BC either. Roughly twice as big, but quite a bit slower and similar BC, so they don't really ever start "passing" the 556 at distance.

    I tossed some JBM calculations into my comparison spreadsheet. Here's a breakdown of my rough calculations. To keep it even, I used Sierra Matchkings for all calibers, and kept bullet weights in the "suitable for mil-spec use" category of "middle of the road". I estimated 16" bbl velocities using the great and powerful google...

    JBM Inputs below (standard atmosphere box checked, same conditions, and 2000 ft elevation). Only thing I changed between modeling runs was bullet and velocity.
    1) 556, 69gr SMK, 2700 fps (from my own rifles' hand load data)
    2) 6.5 Grendel Beefy!, 107 SMK, 2600 fps (added ~100 fps to some interweb reports from 16" bbl Grendels)
    3) 6.8 SPC, 115 SMK, 2500 fps (interweb reports of 16" bbl velocities)
    4) Real 6.5 Grendel, 107 SMK, 2500 fps (interweb reports of 16" bbl velocities)

    Velocity in FPS
    New Military Velocity.jpg

    Drop in inches
    New Military Drop.jpg

    Energy in ftlbs
    New Military Energy.jpg

    Wind drift (10 mph, 90°) in inches
    New Military Windage.jpg

    Based on my calculations, it seems that if they could make a "beefy" 6.5 Grendel, that might be the real ticket. If you add about 100 fps to published 6.5G speeds, it's equal to the 6.8 SPC in power at the muzzle, and the gap only widens as it goes down range.

    Even the existing Grendel catches the 6.8 around 120 yds out, and exceeds it from there on. Perhaps there's reliability issues in the fat bottomed, skinny necked, Grendel that preclude it from being suitable for military use in dirty, disgusting, poorly maintained, machine guns...?

    I'd think the higher sectional density in the 6.5 would also be well suited for armor penetration, and/or the use of fancy military rounds with weird stuff inside (tracers, incendiary, AP, etc).
     
  3. cohunt

    cohunt Well-Known Member

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    We can all play that "favorites" game-- I have a load for my 16" 6.8spc that pushes a 130 vld bullet at 2700fps, or only works with a few specific bullets but it works.
    I dont think the mil wants either the 6.8spc or 6.5g-- they are looking in whole different arenas. There is rumors of a "mid size frame" AR comming soon that would allow something longer than ar15 mag length, but shorter than full 308 length, but I dont think the mil is looking at it either.
    We can all speculate, just have to wait to see- time will tell if the mil is serious or just testing again.
     
  4. J E Custom

    J E Custom Well-Known Member

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    I was referencing the 6.8 SPC because it was designed as a replacement for the 556 and not knocking the 556. I realize
    that a cartridge can perform differently with/in different rifles

    My 16" barreled 6.8 would not reach the factory velocities listed (2800 ft/sec) until I switched to a 24'' barrel. The designer of the round (Remington) sell loaded ammo that will do the listed 2800 ft/sec with just over 2000 ft/lbs. (2002 ft/lbs of energy) https://www.midwayusa.com/product/1...n-spc-115-grain-sierra-matchking-hollow-point

    The best I ever got from the 556 was 3400 ft/sec with a 53 grain bullet for just over 1400 ft/lbs energy. So comparing the best I could get from both the 6.8 is over 600 ft/lbs better, and on game you can definitely see the difference.

    I totally agree that they should have used the 6.5 bullet and in fact, I had considered designing a wildcat with the 5.5 bullets and the 6.8 cases but the difference was almost nil. so it would not be worth it.

    One other possibility that I was surprised they didn't look at was the 7 TCU (A 223/556 necked up to 7mm) this round was very popular with ISHMA shooters.

    The 762 x 39 ammo is almost all the same because of the weapons made for it and the 6.8 will out do it so to me it is a winner. there are other cartridges that are others favorites as it should be. but my belief is that it would be a good choice to upgrade the 556 and keep the same format of a rifle.

    I am also one of those guys that think that the M-14 was great and should still be in service or at least some variant.

    Good discussion !!!! keep it going.

    J E CUSTOM
     
    Last edited: Feb 28, 2019
  5. BallisticsGuy

    BallisticsGuy Well-Known Member

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    Don't forget that the .223 case has been one-upped in the form of the .204ruger and it would fit in the existing guns and magazines and feed just fine. It's just enough bigger to give back the room lost to larger than .22cal bullets. In the world of metallic silhouette the 7-204 and 6.5-204 got a good bit of attention for a while. Now if you took the idea of a .25 cal 100-110gr or .26 cal 120gr projectile in that case you'd end up with 2800fps from a 24" tube with ballistics right in there with a .308 running 168's and energies twice that of a .223. Even from a shorter barrel, it'll still stomp all over

    The larger bullet isn't there just for BC, it's there to defeat the target. Longer heavier bullets have higher sectional density which punches through targets better. Once you get up to 7mm you really lose start out with the bullets you could reasonably fit in the case and have still have feed from a STANAG mag.

    Were I to be designing a round for a select fire intermediate power battle rifle, I'd be looking at something like a 25x45 running a closed nose .257cal 110gr for the 18" and longer barreled rifles and going to an 80-87gr for the 14-16" barrels. I'd also go to the .204 case from the .223rem case. Set all the barrels up with a 9 twist for compatibility and it should be doable. Keep in mind that such a choice would be predicated on running pressures up around 60k and putting adjustable gas blocks on the rifles so they don't try to rip case heads off when you put the heavy bullets in the short barrel rifle.
     
  6. cohunt

    cohunt Well-Known Member

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    M14 is still used on front lines by marines and coastguard --- also the m21 and m14 are modified m14's
     
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