The 6.5x55 Improved Arch was an earlier IMP design on the 55mm case. IIRC had a 40degree shoulder and would easily make 2,908fps with 140's from 26" or longer barrels. Ackley listed a number of loads over 2,700fps with 160's using twists of 7, 9 and 10" - I assume Spitzer shaped Barnes. One load listed is a 200gr. Barnes bullet at 2,624fps form a 5 1/4" twist bl. That would be a long one.