https://www.quora.com/Why-is-the-USA-mi ... s-to-6-8mmJoe Buettner
translated gun manuals, tested ammunition, compiled gun lawsAuthor has 2K answers and 23.1M answer views1y
Originally Answered: Why is the US military switching from the 5.56 mm cartridge to the 6.8 mm cartridge and what is the timeframe of implementation?
The easy surface level answer is that enough generals obsessed with the concept of “overmatch” agreed that front line soldiers should have a higher performance rifle than an M4 Carbine that they approved procurement of one and initial procurement will last into the 2030s. But once you get past the surface level, things start to get… questionable.
There will undoubtedly be overruns on the timeframe, because arms procurement programs of any noticeable size always have overruns on cost or delivery time, that’s normal. But the why is a huge can of worms. Officially, only units expected to go into combat will be issued with the new rifles and everyone else will retain 5.56mm ammunition and M4 Carbines, which should raise some eyebrows about the logistics for who gets what if there’s a mass mobilization.
The 6.8x51mm cartridge is so high performance that the cartridge casing needs a steel case head for its brass body to cope with the shockingly high 80 thousand PSI chamber pressure. Why is the chamber pressure so high? Well, depending on which part of the program management you decide to believe, it’s either so the 6.8mm cartridge will be able to reach out super far or be able to pierce any body armor an opponent can field before they field it.
Getting into an arms race with future armor is a weird thing to do, but there’s some logic to it because modern body armor is pretty much at the point where any further increases in protection to defeat increasing threats will come at the cost of more weight and modern body armor is already at a weight threshold where it affects mobility because there’s only so much weight human tendons can handle without injury. So presumably a peer opponent gets to choose between having mobile soldiers who will only be protected against fragmentation and immobile soldiers who will be protected against rifle fire. That’s not necessarily a bad choice to force on an opponent, but it’s something that was already the case with armor-piercing 5.56x45mm ammunition like M995 which would be far easier to ramp up production for than setting up entirely new factories for a botique cartridge and rifle combination nobody else uses. Now, maybe there’s some side benefits if the 6.8x51mm is able to shoot through the side armor on BTRs and BRDMs and it seems like that’s within the realm of plausibility, but if 6.8x51mm is able to do that then absolutely nobody is talking about it, possibly because planning to ambush armored vehicles with rifles is a bad plan when proper anti-tank weapons are at hand, which they probably ought to be for the units which will be issued the new rifle and cartridge.
The range argument is weird too. On the one hand, the new rifles will be fielded with an optic that incorporates an integrated rangefinder and ballistic computer that will make delivering effective fire out past 500 meters a doable task for the average soldier. The optic appears legitimately excellent and will almost certainly be picked up by other US military branches beyond the army. But the need to deliver effective fire beyond 500 meters is not on average a normal thing to do. It kind of was enough to be notable in Afghanistan, but it’s hard to get clear sight lines on enemies you can positively identify everywhere else. Ukraine has averaged 400 meters and shorter, which is much more normal. Any urban conflict trends towards 50–100 meters and shorter. At all normal combat ranges, 5.56mm ammunition retains more than enough velocity on impact when fired from normal length barrels to shoot people dead and the 6.8mm just isn’t needed.
Speaking of weight, the new rifle and its ammunition are considerably heavier than M4 Carbines and their 5.56mm ammunition. That’s really not desirable at all since weight related musculoskeletal injuries were already a leading cause of medical evacuations in recent US conflicts and having tendons snap by carrying too much of your own gear just… isn’t good. You simply can’t train harder to be stronger to carry an extra few pounds when a combat load is already averaging 20–60 pounds over the weight limit for everyone, no matter how much your CrossFit instructor recovering from a tendon injury claims otherwise.
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Despite all evidence of the Second World War and Korean war showing that infantry engagements were always within 200yards, the US Army Generals insisted the rifleman needed to be able to one shot a charging cavalry horse at 1000 yards.
They then went to Vietnam, where most engagements were within 50 yards, and reacted, or over reacted*, by rapidly replacing the 7.72 round with a 5.56, I’m not sure if this was a deliberate “No, the 7.00 is still too big”.
The M16 5.56 gave good service until Afghanistan, where 1,000 yard engagements were the norm, and, for political reasons, the correct weapons for returning fire at 1,000 yards were not allowed.
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