I dunno, guys... I guess the Ackley improvements might have been useful back in the early days of Winchester and Peters (etc.) .348 bullets... but more recent advances in bullet design and construction over the years -- like bonding, partitioning, jacket engineering, skiving, etc. -- all seem to me to make the original .348 case shape good enough. And all those current bullets are occasionally available from Woodleigh, Swift, Hawk... and even Hornady and Barnes offer some decent ones, too.
In my case, with an original family pass-down M71 from 1937... all the discussion about reboring and rechambering... isn't ringing my chimes all that much. Of course, there are no grizzlies or brownies in my back yard.
Another few FPS doesn't seem all that important to me, these days. (That's pretty much across the board, not just an M71 thing; if I NEED more FPS, maybe I need a bigger rifle.)
Like 1AK, I suspect Winchester wanted to go "bigger" than their previous .33s, and wanted to differentiate from those... and maybe the non-history (?) of .35s was an influence.
From this distance, I'm kind of surprised they didn't think to also introduce a .411 or some such at much the same time. Not a .405, not a .416, given those names were already taken... but a counterpart big-bore version on the Model 71 platform. Mere speculation about whether that could have succeeded or not...
FWIW, I'm going out for whitetail next week, expecting to carry the .348 most of the time. I'm guessing it'll be "enough gun" for the task -- if I were even to see a deer and get a shot.