American or European, no manufacturer is going to stray too far from SAAMI specs.
The spec length of an unfired, folded crimp, 2 3/4" 12 Gauge shell is 2.405", tolerance of +0/-0.250".
The spec length of a roll-crimped shell is 2.450", same tolerance. The spec length of a fired shell is 2.760", +0/-0.100".