I considered a straight wall 35 caliber 223 case; the resizing die situation would be easy (357 Max), but headspacing on the case mouth can be a picky affair in long straight wall cases at 5.56 pressure. I'm not sure how well it would feed in an AR anyway.
There is a website dedicated to just such things. Sorry, can't remember what it was called, but the consensus among those who tried it was that simply necking up a .223 to .357 had 2 problems: not enough shoulder and too long to feed.
In my limited experience with the BLK I've been getting tremendous case stretch when I push loads.
