It's designed to work in a standard NATO 556 magazine, receiver and bolt carrier, the cartridge body is the same as a 556 NATO, it's necked to take a 7.62 bullet.
https://www.youtube.com/watch?v=MTZRCEh1Czg and https://www.youtube.com/watch?v=f4gst0QoMw4
the blackout is also unique in that the subsonic version of it was designed from the start to be NATO-rifle-platform compatibile with a suppressor. and due to even lower velocity, even shorter range (definitely 75 yard and less).
also fairly unsure on its performance vs. modern plate based body armor vs greater velocity of regular 556 nato, 6.8 as you mention, etc.
The new 6.8 is a much heavier and higher pressure round than the 556 and delivers far greater range and body armor defeat capability. The pressure is so great (80,000 psi, 551.6 MPa) that the case head (the 'back' part where firing pin hits) is made of stainless steel instead of brass; a 'composite' (steal + brass) case. I don't think that has ever been done in a military infantry weapon; all steel cases have been around a long time, but those are about cost saving as opposed to performance.
80,000 psi is pretty extreme in a rifle round. The high end of traditional brass case ammunition is 65,000 psi. That includes high performance rifle calibers like 338 Lapua Magnum and 22-250.
Also, the XM157 Fire Control optic being built for these new rifles is remarkable: "variable magnification optic (1X8), backup etched reticle, laser rangefinder, ballistic calculator, atmospheric sensor suite, compass, Intra-Soldier Wireless, visible and infrared aiming lasers, and a digital display overlay."
Due to the undoubtedly high cost these weapons will be limited to the 'first responder' combat forces and M4/M249 will be the mainstay for the rest for many years yet. But this commitment including the official designation of a new cartridge is a clear signal that the DOD is taking infantry weapon performance rather seriously.
[1] https://www.army.mil/article/255827/army_awards_next_generat... "The XM5 Rifle will replace the M4/M4A1 carbine within the close combat force, and the XM250 Automatic Rifle is the planned replacement for the M249 Squad Automatic Weapon"
It is pretty extreme but I can't say I'm exactly surprised, metallurgy has clearly advanced quite a bit since the days when 556 NATO pressures in chamber were standardized 50+ years ago. If you have DoD budget levels of money to spend on a barrel you can clearly do higher.
Barrels that meet 556 NATO spec are as cheap per unit as $100, a really nice one is $300. If it's not a problem that your barrel alone might cost $700 I can see this being totally possible.
The 6.8x51mm will not defeat any armor that M2AP won't. NIJ certified level IV armor will stop M2AP, 6.5 Creedmoor, .300 Win Mag, and in some cases, up to .338 Lapua Magnum. The last two are far more powerful, with greater velocity and sectional density, than 6.8x51mm, and they still won't reliably penetrate with a single hit.
Armor penetration was a stated goal of the Army's NGSW program, and the new cartridge will penetrate better than 5.56, but it's still not going to penetrate modern level IV armor.
in some of the DIY-drone-strike footage I've seen recently it appears that the russians are wearing no plates at all.
A .300 blackout, will 'fit' into a .223; or 5.56 NATO chamber, AND allow the bolt to go into battery position.
cataclysm results:
https://www.thefirearmblog.com/blog/2018/06/19/lfd-research-...
https://rifleshooter.com/2019/06/kaboom-300-blk-in-5-56-223-...
most current AR chambers are .223 / 5.56 WYLDE chamber meaning both will fit and function.
the problem comes from a straight up old school .223 being fed 5.56.
the evolved gas pressure is higher for the 5.56 round, and will probably work for a while, will put extreme use on the rifle and will eventually malfunction probably with similar results to the .300BLK problem.
a 5.56 can be forced to chamber in a 7.62 chamber, but is about 6mm too long so you will know theres a problem
7.62 nato of standard configuration wont fit the lede [ogive] of the 5.56 chamber unless you shove really hard with a mallet.
what you may see is a lower receiver stamped for 5.56 NATO assambled with a 7.62 upper and barrel. this is proably fine for most aluminum lowers but i would stay clear of the polymer lowers or the skeletonized milled out lowers, for the 7.62
the last paragraph is not exactly correct, a lower receiver for 556 NATO STANAG magazines has a different length magazine well (AR-15 derived design, not AR-10) and cannot take the dimensions of an AR10 magazine/7.62 NATO magazine.
you can mistakenly chamber a .300 blackout in a 556 nato rifle upper because the blackout is the same length cartridge as a 556 nato round and will feed.
you cannot mistakenly shove a 7.62x51 NATO/.308 winchester round into a rifle lower/upper that is not an AR-10 derived design because there would be no way to even seat the magazine.
even if you were to somehow mistakenly mount an ar10/762 NATO upper and barrel on an ar15 lower the only way you could try to get a round into the chamber would be to shove it in sideways through the ejection port
>>what you may see is a lower receiver stamped for 5.56 NATO assambled with a 7.62 upper and barrel<<
5.56 magazine wells can be modified to fit 7.62 magazine there a couple examples on the range right now, also a .375 win magnum necked down to a .223 projectile.
up here its common to ammo up with a full mag, +1 one in through the out door
if you have a photo example of someone actually doing that on an ar15 magazine well i'd be very interested to see what the hack job looks like
why would someone actually chop a lower? because its not available, or its a do something winter project to chase off the cabin fever.
I think 300bo is closer to 9x39mm in application.
https://news.ycombinator.com/item?id=32389075
it doesnt fit the bore of the barrel, it 'fits' the chamber.
if you are using .300 blackout and .223/5.56 nato be very carefull or do something like colour your rounds with a sharpie
[edit] by the way there are very few dumb questions, asking a question is far more intelligent than acting like you know it and blundering through it by luck.
there is a color coding system for ammunition:
https://www.bulletpicker.com/pdf/MIL-STD-709C.pdf [PDF]
indicating special function, such as armor piercing; incindiary; fragmentary HE etc.
it is also a popular option to have receivers colorized or stamped. alot of magazines are polymer and could easily be colorized, and detailed, for instance with a large pictogram of the intended ammunition.
this is also on a little more of an angle, a reason to not let that combination of, persons you dont trust; .300 blk; and 5.56 chambers to occur.