I saw an article that said the military is going to a 6.8 caliber rifle in the near future, made by SIG, I think. I wonder if the cartrige is of a comarable size so that an AR- 15 lower could accomodate an upper with the 6.8 cal. ? Just wondering.

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Kevin in Oregon

Sig sold them a whole weapon system, new different rifle and cartridge system
Army chooses Sig Sauer to build its Next Generation Squad Weapon

It's not the AR friendly 6.8 SPC cartridge. More like an AR 10 but different.

I'm reminded of something Clint Smith said "Reinvent the wheel and name it after yourself." He was talking about trainers, but it fits.

I read about it several months ago. It has an operating pressure of 70,000 plus PSI. The case is a two piece one with a steel head and brass case (I think, maybe plastic). Pretty hot stuff! I've never been an AR .223 fan, but really? Double up on the hearing protection!!

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Richard Gillespie
FBINA 102

.277 Fury - Wikipedia

A cartridge operating at 80000psi in an AR10 platform. What could go wrong? The cartridge is 51mm long which is the same as the 308 Win so the answer to your question is, no, it will not work with an AR15 lower. In fact, I'm not sure what kind of vodoo SIG is using to get an AR10 lower to stand up to that kind of pressure, but I'd be a little leery about using a regular civilian AR10 to shoot it.

The lower doesn't see a whole lot in the AR design. The heavy lifting is between the barrel, BCG and buffer assembly.
3D printed lowers of the cheapest plastic man has contrived can hold up to the impulse of full power 5.56 and 7.62.
It wouldn't take much to bring an AR up to the task if it isn't already.
Now that barrel and BCG ... that's what has my interest piqued

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it just needs more voltage

The bolt thrust on that thing has to be off the charts. It will, in time, wear on the retaining pin holes in the lower. I've also seen them crack at the rear of the lower where the back pin of the upper is. And that was in 5.56 guns. I have not seen one of these guns so I have no idea but it would surprise me if SIG didn't alter it somehow to take this into account. 80000psi is going to stress everything in the gun. That's 20000psi over the max NATO specs for the 7.62x51! Of course it's the Military and this is a plug and play gun so replacing barrels and other parts isn't an issue for them but I wouldn't want to have pay for it in a gun that I own. The Army has about 100000 that will be going into the field in 24. It will be interesting to see what ultimately ends up happening with them. I'm not convinced that they will be a long term general issue battle rifle but we'll see. The up side with this system is that if the 277 doesn't work out, it won't be a big deal to change them to 6.5 Creedmore or 308.

I'd think that to be more an issue of fit between the receivers.
Sloppy fit will act similar to rod knock in an engine.
None the less, I suspect some flavor of steel reinforcement is probably incorporated into the buffer tower at the least.
I'm more concerned with the barrel itself.
If the 62kPSI max 220 swift got a reputation as a barrel burner, 80kPSI is saying hold my beer.
In a Tet offensive rematch, which bricked a good number of M16's, I imagine the bunkers and trenches will be well illuminated by the glow of all these scalding hot clubs.

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it just needs more voltage

Definitely on point. The fit issues can be addressed and the wear issues too, everything can be addressed if you throw enough money at it, but you have to wonder what is going to happen when cheep uppers start appearing on the market. I don't know what the barrel life is expected to be but the Army will just replace them. Expense is the last of their worries. I do understand that they will be loading two rounds. One in a regular brass case for training that is loaded down and the combat round in the hybrid case that runs at top pressure. It's supposed to replace everything but how many times have we heard that?

Yeah .... Since before the OICW program.
If they manage to make it run at 80k, that's going to require some next level metallurgy that will be of great value to us in future arms

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it just needs more voltage

...being the length of a 7.62 NATO and runs at 80k psi I wouldn't call it an 'intermediate' cartridge by a long shot (pun intended) For some reason the Ordinance Department never listens to me, but I think that they've done the opposite of the 'too light' 5.56 and gone to what amounts to a good replacement for the 7.62.
Geez people.....

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"He was kinda funny lookin'"