The first place to begin your quest for understanding cartridges is the cartridge specs. Variations in bullet diameter, case capacity, and pressure give us insight into what these cartridges can do on the range, in self-defense situations, in hunting, and even in battle.

As you can see in our comparison, the .277 Fury is a bit larger than the 6.8 SPC (6.8x42mm compared to 6.8x51mm). But the most notable difference between the two cartridges is the massive difference in SAAMI chamber pressures at 80k psi versus 55k psi.

At the time of writing, the 277 Fury has the highest approved chamber pressure of all time, even outstripping the 338 Lapua Magnum and 50 BMG.

The increase in pressure means the .277 Sig Fury will have a higher muzzle velocity and more energy than the 6.8 SPC could ever dream of. Sig easily defeated Remington in case specs, even in rifles with shorter barrel lengths.

https://ammo.com/comparison/277-fury-vs-6.8-spc

I don't own a weapon in either of these calibers, but I have watched for a few years the arguments between these in an effort at adoption. So here's a good article for those of us interested in such things.

The difference in pressure is astounding to me.

CDFingers

Of course the .277 Fury is going to be more powerful than the 6.8 SPC.

First, it's a larger cartridge. The 6.8 SPC was designed and sized for the AR-15-size action. The .277 Fury is the .308 Wincheser necked down to 6.8mm, much like the 7mm-08, and sized for an AR-10/LR-308 action. That wildcat's been around for a long time, but was never that popular and therefore not "legitimized" as a factory adoption.

Second, the military version is loaded to insane (80,000 PSI) pressures, something that I would try only in perhaps a Weatherby Mark V action (they've been tested to 200,000 PSI). That's why the military version needs that stainless-steel case head. Were we to load a 7mm-08 to similar pressures, it would outclass even the .277 Fury at distance due to the 7mm bullets' really amazing ballistic coefficients, even over 6.8mm, and that's saying something. For that very reason, I'm rather surprised that the Army didn't specify the existing 7mm-08 and simply adopt a higher pressure version of that cartridge.