6.8 Bigfoot aka 6.8x42mm. The case length and bullets of the 6.8SPC with the larger diameter brass of the 6.5 Grendel. 200 fps faster than the SPC. Short enough case and still have the powder capacity to shoot almost all of the 270 cal 130s around 2700 fps. Runs in 6.5 Grendel mags and bolts.
Start with long 7.62x39 (blanks w boxer primer) brass, cut it to length and fireform in chamber. Resize in dies and load.
Another option is to make it the same length as the 6.8SPC so current SPC barrels can be converted. Shorter 130s will still fit. Rechamber the SPC barrel, swap to the Grendel bolt and it's done.
Heres a pic of the Privi Partizan 7.62x39 long blank brass. Perfect diameter, rim and groove size to fit the Grendel bolt head.