Originally Posted By Bushman_269:
They share the same bore and bullet diameter. Nothing else is the same.
The .270 Winchester is based of the .30-06 case, which as you may know is a good 2.494" long for a Overall Length of 3.340, which we consider a magnum length cartridge for use in a long action gun. It has a powder capacity near 64 grains or so.
The 6.8 SPC is based off an obsolete Remington cartridge, the .30 Remington. Its case is 1.686" long for an OAL of 2.250", which is a short action length case intended for feeding through AR magazines, as the .223 remington is also 2.250" long. Its powder capacity is somewhere around 32 grains or so, half of what the .270 Winchester can hold.
The .270 Winchester can shoot bullets from 85gr up to 150gr, up to 3400fps for the 100gr loads and up to 2900fps for the 150gr loads.
The 6.8 SPC can shoot bullets from the 85gr (TSX) at around 3000fps in a properly spec'd barrel, and up to 2450-2500fps for the 130gr bullets in same barrel, but generally speaking using the heavier bullets will require the bullet to be seated further out of the case and thus eliminates the ability to feed from a magazine.
Bottom line, .270 Winchester gives you 'magnum performance', (velocities from 2800fps-3000fps on up) in bullet weights from 85gr-150gr whereas the 6.8 SPC can only reach such speeds with the absolute lightest offerings in .277 dia.
.270 Win is a big long case, suitable for bolt action rifles and long-action semi autos, the 6.8 SPC is a very short case intended for AR-15 magazine dimensions.