I’ve read that a rebore requires .030” over groove to work reliably.
Of course, a 318WR (.330” groove) to 35 Whelen (.358”) is actually .028” difference but seems to work out well.
My guess is that the barrel is drilled out a couple thou over groove, maybe more if there’s pitting so a 318 would end up with a bore of at least .333-.334 so that is why one cannot be rebored for the 333 Jeffery.