Jason, thanks also for clarifying my query about not having to worry about manually feeding the relatively straight-walled 6.5x55 Improved case into my old B78’s chamber. I’ve often stated that the members of this particular forum are fantastic for openly sharing their relevant knowledge and experience – keep it up !
Savage99, according to the following article, (http://shootersforum.com/showthread.htm?t=5813
) some cartridges are ‘improved’ more so than others when their shoulder angle, taper and powder capacities are altered. Apparently the 6.5x55 has the 4th best velocity gain - supposedly offering a 13.7% increase over the standard factory round with a 140 Gr projectile.
In contrast, “Right on the borderline of our rule of thumb for getting involved in is the old-time .219 Zipper, no longer being chambered for. The factory load with a 55 gr. bullet is 3300 fps, and the Ackley can achieve a velocity of 3500 fps, a gain of 6.0%. This is a rimmed cartridge and just about obsolete, with very little demand”.
For consideration, apparently standard US factory loaded 6.5x55 cartridges are purposely loaded well within their maximum powder capacity in order to be safely fired in the old ‘weak’ M94 type military rifles. To be fair, the Improved 55’s rather substantial 13.7% velocity gain probably stems from a near max powder load fired in a long barrelled modern ‘stronger’ action rifle.