canon6 (Doug)
I went to the forum that you reference and it lists this as being from a 243 Win.
In a handi the improved version would in all likelihood be longer than the 243 Win Ackley Improved to the neck/shoulder. I would think that overall, it would be similar, just slighlty longer, and as such would be a pure wildcat. (The 243 Win AI will allow the use of regualr 243 Win, something that is problematic in the Handi as you spend lots of money to set the barrel back.)
Looking at the prices there, it would be as cost effective to buy a barrel that would work for a re-bore if you wanted something that NEF does not offer. See Fred M's, Mac11700's and quickdtoo's projects (257 Roberts, 338-06, and 338-06 and 405 Win., respectively. Or start out with a barrel of desired dimension but in a smaller chamber to get what you want. IE 22 Hornet to go to 223 Rem AI or 225 Win or 223 Rem to 225 Win.
stevespages and reloadbench have cartridge dimensions (reloadbench is offline for reconstruction at present.)
I look forward to seeing what David White uses for dimensions on the 243 Rimmed Improved.
Neal