Home Forums Hunting & Shooting Hunting Rifles 280AI vs 270 Winchester

SAAMI MAP specs are one difference:
6.5x55 = 51,000psi, 46,000 CUP
6.5 CM = 62,000psi, no US CUP standard

Cartridge length is another:
6.5xx = 3.150"
6.5 CM = 2.825"

Within pressure standard limits, achievable velocities for 129-130g bullet are about the same according to Hodgdon:
6.5x55 = 129g @ 2827fps, 45,300 CUP (98.5% of SAAMI MAP)
6.5 CM = 130g @ 2735fps 59,500 psi (97.5% of SAAMI MAP)

The 6.5x55 with long/heavy bullets is best used in a mid-length (7x57mm) or long (.30-06) action while the CM was specifically designed for a short (.308 Win) action length.