Just how much damage could be done to the rifle and the shooter if a 5.56mm was fired in a rifle chambered in 6.8mm rifle?  What I was thinking about was, in a firefight where a squad had both rifles, what would happen if someone inadvertently put a magazine of 5.56mm ammo into a 6.8mm SPC?  Bullet rattles down the bore, piss-poor accuracy but no one's hurt, or the gun explodes, rifleman injured?  What about the reverse of trying to chamber 6.8mm in a 5.56mm rifle and jamming the gun, rendering it inoperable?  Are magazines chambered for 6.8mm modified so they won't fit into the mag wells of 5.56mm guns, and vice versa?

Quoted: The XM8 is still 5.56mm. -- Chuck

So does that mean the XM8 was accepted & be replacing the current 16/M4.

I wish this 6.8 thing would just GO AWAY.

I don't think the 5.56 cartridge would even be held in place enough to fire -- I bet it would slide forward until it no longer touches the bolt.

-z

Quoted: I wish this 6.8 thing would just GO AWAY.

Not likely..

I'm with zak on this one.  Would it fire?  How would there be any pressure on the pin and primer.  It wouldn't even likely be in straight enough.   What is your point anyways?  You act like the military has never had more than one round!!!  Could someone mistakenly try and load an ak74 with 5.56.  Sure but for crying out loud.  Has anyone ever tried to fire a 20 gauge shell in a 12 barrel.  I don't know.  Has anyone ever won the darwin award for strapping jado rockets to the roof of their car and sqashing themselves into the side of a mountain?  I don't know.  What is all this one round  crap.  If I'm not mistaken their may even be a few in uniform that carry a1's.  Albeit they're not front line.  But do they know the difference between m855 and m193?  Well that big green tip is a start.  I'm not trying to be mean but you sound like a newsman that has to tell us prolitarins how to live otherwise we'd probably kill ourselves we're so stupid.!!!!!!

Okay, I'm over reacting.  I suppose because the rifles  look identical and the mags too this MAY be a concern.  I'm sure they'd come up with something to help them remember though.

Go JJ!

Quoted: I'm with zak on this one.  Would it fire?  How would there be any pressure on the pin and primer.  It wouldn't even likely be in straight enough.   What is your point anyways?  You act like the military has never had more than one round!!!  Could someone mistakenly try and load an ak74 with 5.56.  Sure but for crying out loud.  Has anyone ever tried to fire a 20 gauge shell in a 12 barrel.  I don't know.  Has anyone ever won the darwin award for strapping jado rockets to the roof of their car and sqashing themselves into the side of a mountain?  I don't know.  What is all this one round  crap.  If I'm not mistaken their may even be a few in uniform that carry a1's.  Albeit they're not front line.  But do they know the difference between m855 and m193?  Well that big green tip is a start.  I'm not trying to be mean but you sound like a newsman that has to tell us prolitarins how to live otherwise we'd probably kill ourselves we're so stupid.!!!!!!

Since there have been cases of troops with M16A1s being issued M855, this isn't some off the wall concern.

Not everyone in the military is a firearms enthusiast.  More than a few would have a hard time even recognizing what the green tip on a round of M855 means, let alone whether or not they can use it in the rifle they've been issued.

Add to that the conditions under which a weapon may be used: in the dark, under tremendous stress, while exhausted.

Making weapons and ammunition as "soldier-proof" as possible is a good idea.  In the case of the M16A2 and M855 this wasn't handled as well as it could have been.  If the 6.8mm is adopted, it's certainly possible that someone will try to use 5.56mm NATO in a 6.8mm rifle.  It'd be a good thing if this resulted in neither a catastrophic accident nor a hopelessly jammed weapon.

Actually..
The latest ish of Guns & Weapons for LE (I think...might be a different rag w/ similar name) has an article about the Barrett 6.8mm.. The only malfunction the author reports is exactly what we'retalking about- a 5.56mm round mixed in with the 6.8.. He states that it did fire but did not chamber the next round IIRC..

well, ahab, if that is correct I stand severely corrected.  However, I don't understand this notion that soldiers don't know the difference between m855 and m193 and an a1 and an a2 and an fn saw or the bigger .30 cal fn and the giant .50 cal and the .30 cal m-14 and the sniper rifle.  Okay Okay, it's obvious that the different type guns have a different type of bullet.  And it's not as obvious if the gun is the same, but again, I'm sure soldiers are trained on what bullet goes with what gun, right?  To say more than a few have a hard time recognizing what a green tip means does not sound right to me.  But I've been wrong many times before so maybe I am again.  It's very elementary though.  Same thing with the difference between the 6.8 and 5.56.  Seems pretty simple to me.  

Quoted: well, ahab, if that is correct I stand severely corrected.  However, I don't understand this notion that soldiers don't know the difference between m855 and m193 and an a1 and an a2 and an fn saw or the bigger .30 cal fn and the giant .50 cal and the .30 cal m-14 and the sniper rifle.  Okay Okay, it's obvious that the different type guns have a different type of bullet.  And it's not as obvious if the gun is the same, but again, I'm sure soldiers are trained on what bullet goes with what gun, right?  To say more than a few have a hard time recognizing what a green tip means does not sound right to me.  But I've been wrong many times before so maybe I am again.  It's very elementary though.  Same thing with the difference between the 6.8 and 5.56.  Seems pretty simple to me.

The typical 18 or 19 year old joining the military doesn't have much firearms experience.

They receive weapons training that can range from excellent to abysmal, depending on branch of service, location, MOS, quality of leadership in the training unit, happiness of the instructor's marriage...

They're likely to hear all sorts of urban myths about weapons ("Can't shoot troops with a .50," "The enemy can use our ammunition in their weapons but we can't use theirs," etc).  Many of these myths being spread by NCOs who themselves don't know any better.

Unless they are in Infantry, MP or some other combat arms units they're not likely to spend a lot of time firing small arms, at either a target range or a live-fire training range.  Let alone be issued live ammunition on a regular basis.  The Army, in my experience, is one of the most gun-phobic organizations in existence.  Soldiers are assumed to be incompetent and unsafe, and elaborate rituals are enacted to prevent the chance, no matter how miniscule, of someone doing something stupid or criminal with a weapon.  I've run ranges for units from other services, and while they're not as phobic, the overall level of knowledge/skill/competence was no higher...  Initiative and true understanding of the subject are suppressed in favor of dumbed-down by-the-numbers training designed not to make soldiers good shooters, but to get the maximum number of troops "qualified" in the minimum amount of time and at the minimum cost in ammunition.

In the case of M855 versus M193 it's quite simple for someone to make a mistake.  The average troop doesn't know much about ballistics beyond "I put the cartridges in the magazine, and the magazine in the rifle..."  Bullet weight and twist rates are a complete mystery to him.  If the supply sergeant for a unit with M16A1s doesn't know the difference and just orders some "5.56mm ball" there's a very good chance he's going to get M855.  I believe there was a case of this cited here a few weeks ago, involving an Air Force unit with 'A1s being issued M855; they had probably drawn their ammunition from an Army ASP, and it's very likely none of the soldiers or junior NCOs there had ever even seen a round of M193 during their time in service.  "They ordered '5.56mm ball' we'll give them '5.56mm ball'" and a pallet of M855 was put on the truck.

Confusing 5.56mm and 7.62mm, when you're holding both in your hands, on a nice sunny day at the range, is difficult.

When it's 0300 and you haven't slept more than 15 or 20 minutes at a stretch for the better part of a week, you've been hauling 50 or 60 pounds of armor and gear around in 100+F heat for days on end, and people are shooting at you and blowing stuff up around you, it's very easy to grab a box of ammunition and try to shove it into a magazine without it registering that it's the wrong caliber.

I read the same article Ahab mentioned. The 5.56 fired in the 6.8 chamber, but didn't extract. Case had to be poked out with a cleaning rod.

agc, your points are well made but scary to me in the least.  I've never been in the military.  I have a cousin that was a raven for a period in the air force.  And he had been a hunter all his life.  But saying that he is on the slow side of things when bullets are concerned.  He now hunts with an sks and thinks it's as good as a .308.  He claims he can shoot it accurately out to 300 yet I'm pretty sure he's never tried.  He has however taken an antelope at 300 with a ruger .223 varmint rifle.  When I asked him what bullet he used he didn't know.  It must've been a lighter bullet because them rugers have a 1/12 twist.  What my point is I'm not sure.  I guess my point is, I can see where you're coming from, although I thought basic weaponry was taught in basic training.  I've never heard of guys confusing ammo before, even on the battlefield.  But of course, I can't know everything.  But the real point is, you would think the best trigger pullers in the whole world ought to know a thing or two about the weapons they use, But I guess maybe that's saved for the more elite units?????  Yet I'm sure some regular ole soldiers would get on here and prove either you or me wrong, eh?

Just read a test on the Barrett. Author fired a 5.56 through it. Bullet hit target, brass split, stuck in barrell, had to be tapped out.