Jeff,
COL can be anything any handloader wants to make it, within the contraints of case length. But unless you build a custom 6.5x55 on a short action AND with a shorter throat, there's no advantage in chambering the 6.5x55 in a short action.
The SAAMI OAL length was set due to the vast majority of 6.5x55's being Swedish Mausers, the reason the SAAMI 6.5x55 throat is also quite long. The standard 6.5x55 reamsers used by every American manufacturer have this long throat.
This is also the reason that SAAMI set the OAL length for the 6.5/.284 at 3.3"--the "Norma" length. If they had chosen to go with the short throat of the "Winchester" 6.5/.284, then Norma's factory ammo wouldn't fit due to the short throat.
The reason the 6.5/.284 came into existence at all is target shooting. It allowed 140-grain bullets with very long ogives to be seated with the shank of the bullet in the neck, while the short throat of the Winchester version forced such bullets to be seated with the ogive inside the neck. Plus, the longer seating allowed about another 100 fps more velocity, but the main reason was the seating problem with long-ogive target bullets.
Hunting bullets, on the other hand, can "comfortably" be seated to the "Winchester" OAL, with any such ogive/shank problems. This is why most 6.5/.284 sportinge rifles are built on short actions, to save weight.
This whole conflict between OAL and the modern short action is why the .260 Remington and 7mm-08 came to be in the first place. Yeah, you can fit a 6.5x55 or 7x57 round into a 2.8" magazine by seating bullets deeper--but then you run into conflicts with the standard throat. So the .260 and 7-08 were developed to resolve the conflict.
I once had a Model 20 NULA chambered for the 7x57, and while the magazine prevented me from seating bullets out to the SAAMI 3.065", it worked fine and I couldn't see any practical difference in velocity. But the throat was a little too long for the finest accuracy, so I eventually had the rifle rebarreled to .257 Roberts AI. The case is exactly the same length as the 7x57, but the standard throat is shorter.
