A bigger issue than chamber length is the angle of the forcing cones. Many older guns have steep angle forcing cones for use with fiber wads and paper hulls. Relieving the angle of the cones will allow the chamber length to remain as is.

As to the length of some European loaded ammo, it may measure 67.5mm instead of 70 mm (2 3/4"). Example--BP F2s measure 67.5mm. As many black rem sp.