So I'm one of those guys loading 277 bimetal cases necked up then down to 308 and 6.5 Creedmoor. The Creedmoor got over 3124 fps average with 140 gr bullets, and over 3000 fps with 150 gr SMK in a 24" barrel. The 308 has some improvements also, but it's a 30" 8 twist Bartlein barrel so I used another 9 twist 22" Bartlein. The 6.5 Creedmoor is a factory rifle and I ordered a 26" 7.5 Twist Bartlein for it ...
I could order a 277 reamer ...but this is an expensive experiment already. Ran through a hundred rounds, check throat erosion and lug set back...it's okay so far. Even pulled bullets on some loads. This bimetal case has promise for a bunch of cartridges. Useful powders seem limited, as is my willy-nilly testing, and it seems logical that barrels will erode faster, so I fired some in fast secession, and check, as this barrel will be replaced, on this sacrificial rifle. I do not recommend doing this...and no manufacturer will stand behind this kind of abuse of their products, as expected. Sig does not recommend reloading these cases or stand behind their 277 rifles with reloads. If you could even find one in 277....I have reloaded some 3 times, in testing so far.
Not recommended by anyone, play here at your own risk. Pressures and its signs are different as are powder reactions to the SS case head as less thermal energy is transfered to the SS case head and more to the powder charge.. I do not believe you can blow a primer out of this SS case head, without destroying the rifle, so don't go there... For most I'd recommend them to buy the brass Lapua cases in these calibers, they work well at slightly elevated pressures.