John,
I understand that Winchester/Olin designed the 284 to fit the magazine box of Winchester 88s and 100s, so a 6.5-284 wildcat was never part of the equation. I think that a reloader's definition of "comfortably", as you have used it, would directly impact what weight/length bullets that reloader would select for his short action 6.5-284. As you know, some folks feel that any portion of the bullet that protrudes belong the level of the base of the neck is a very bad thing, while others claim that since you're not likely to fill the 284 case to 100% capacity, the protruding bullet doesn't have any impact at all on potential velocity performance. Is there a clear right or wrong on this issue, or is it "my way is better" objective "measurement"? Me personally, I don't like to seat my bullets below the base of the neck, but seldom get a case of the vapors if I do happen to select a bullet that requires a little of the base of the bullet to protrude below the neck.
But putting the 6.5-284 aside, you would agree that a 260 built on a long action wouldn't have any COAL issues that would limit the make/model of bullet used, so that the only difference between a 260 in a long action and a 6.5x55 in a long action would be the 2 grains difference in case capacity? The 6.5x55 would be a suboptimal solution in a short action, because the COAL limitations of the magazine and the longer SAAMI specs throat would directly influence the length of the bullet used and the amount of freebore, but the 260 in a long action wouldn't be a suboptimal solution if the barrel was throated for the longest bullet that builder wanted to use? Would it?
JEff
PS - The corollary of this thread is the ageless "How many spirits can dance of the head of a pin?" debate.
