7.62x39 has pretty extreme case taper, whereas Grendel doesn’t.
The SPC would have been great for military application in .257”, with mags that take longer COL, 30˚ shoulder, AerMet bolt or CNC liquid burnished bolt and extension, and higher BC projectiles in the 110-125gr region. It would not generate the same amount of “slap” on target within closer ranges that the .277” and .30 cals do, but killing performance would still be the same as any other medium bore 110-125gr, projectiles construction being more influential at that point.
The 130gr Berger Classic Hunter with .490 G1 BC/.251 G7 is where it’s at in the 6.8 if you’re wanting it to compare well with 6.5 Grendel. Load that in PRI or Six8 mags as long as you can and use the same powders known to perform in Grendel, and it’s a great intermediate-to-long range load, provided you have tighter twist.
The DoD entities who drove what became 6mm ARC were looking for long range performance over the 77gr, so Mk.262 has always had its limits. They just got tired of dealing with all the penalties of the SR-25/7.62 NATO weight/bulk/limited mag stowage, recoil, and muzzle blast.
They actually ran through the initial test iterations with 12-18” Grendels, and said for every shooting scenario they employed them in, there wasn’t a single instance where they would have preferred SR-25s. Hornady then said they could tweak it to make it better, and necked it down to 6mm. It would have been easier to dust-off 90-110gr class of bullets and optimize the BCs on them like they just did with the 100gr ELD-VT 6.5mm.
If you shoot 77gr even from 24” bolt guns next to a 12” Grendel on the same targets at distance, you quickly see that the juice is worth the squeeze. 77gr barely nicks the paint, whereas you see the typical bullet splash and base dirt/mud linear base-of-target splash from 6.5mm.
