Only the shortest of varmint bullets would work.
In fact, the 6.8SPC popularity has spawned a whole new generation of .277 bullets with short ogives that didn't exist when the 6.8SPC was first introduced.
From my Black Hole longer barreled 6.8mm AR, some of the traditional .227 caliber bullets work just fine. I use 1,000 ft pounds of energy as my minimum hunting impact energy for things like Mountain Goat, Dall Sheep and Caribou
130gr Sierra Soft Point flat base...27.0gr AA-22002.280 co,,Vel=2,658 fps and a 1.0 inch group Dirty necks Muzzle Energy 2,039 ft lbs. Point Blank hunting range of 263 yardsHolds 1,000 foot pounds out to 425 yards.
130gr Hornady SST BC=.460...27.2gr AA-2200, CCI 41 Military primer2.290 colVel= 2,570 fps and a 2.00 inch group Pressure signs OK, no dings, Ojive had to be crimped close Muzzle Energy , 1,906 foot pounds, point blank hunting envelope to 256 yards Holds 1,000 ft pounds out to 410 yards, fired at 10 degrees below zero
150gr Speer Soft Point Flat Base...25.0gr AA-22002.280 colVel= 2,360 fps and a CLOVER-LEAF HOLE dirty necks, semi-flat primers Muzzle Energy 1,855 foot pounds, Point Blank hunting range 237 yards Holds 1,000 ft lbs out to 375 yards
150gr SPEER SOFT POINT ...25.3gr AA-2200, CCI 41 Military primer2.291 col Vel= 2,286 fps and a 0.50 inch group No pressure signs, clean cases, cycles well Fired at 10 below zero.