Okay, the 30/30 pushes a 150 grain .308 diameter bullet at 2300 f/s froma 20 inch barrel, the 762X39 makes a 123 grain .308-.311 diameter bullet at about 2400 f/s from an 18inch SKS/AK barrel.
The 6.8 pushes a 115 grain .277 diameter bullet at about 2550f/s from a 14 inch M4 barrel.
Add 6 inches to the 6.8 and you see advantages, but the 762x51 is already a mil-spec round with several off the shelf platforms available and in the field.
Calculations indicate not too much difference in KE at the muzzle (30/30 leading), plus the 115 grn .277 bullet has a rather poor BC (about the same as the 123 762, slightly better than the 150 grn 30/30 -because the 30/30 uses a FP bullet) and therefore a short range trajectory.
Correct me if I am wrong.
Comparing the rifle calibers, I was mainly focusing on your lumping the trajectory of the 6.8spc with the 30-30 and the 7.62X39. This is not a correct comparison. Here is one handy table to check:
http://www.chuckhawks.com/rifle_trajectory_table.htm
Here are some common calibers, and their “max point blank range” which is the greatest distance the bullet will travel between 3” above and below the bore. This is a shorthand method for quantifying the flatness of a trajectory, from rainbow to laser.
.223 55gr at 3240fps MPBR 239 yards
.243 110gr at 3100fps MPBR 283 yards
6.8 115gr at 2800fps MPBR 267 yards
30-30 150gr at 2390fps MPBR 225 yards
7.62X39 123gr at 2365 MPBR 225 yards
7.62NATO 150gr at 2800 MPBR 275 yards
7.62NATO 180gr at 2610 MPBR 259 yards
30-06 180gr at 2700 MPBR 269 yards
So you can see, in terms of trajectory, the 6.8spc is up with the 7.62NATO and 30-06, not the 30-30 or 7.62Russian. On the battlefield, this flatter trajectory translates into a much longer effective range. A different table showing drops at longer ranges would show the 30-30 and 7.62 having so much greater drop that they would be categorized as having rainbow trajectories. Not so with the 6.8, 7.62NATO or 30-06.
Correct me if I am wrong.
The AK47 or AKM barrel is usually 16.1 inches long. That of the SKS/CKC Siminov Carbine is 521 MM/ 20.5 inches in lenght. Best of all, the RPK Squad Auto version has a barrel 23.5 inches long, offering both a little better velocity and a longer sight radius than its shorter cousins.