Some years ago I had the original Corbett rifle in hand and discussed the way to reproduce it with Marc Newton. The original is built on a very rare "short intermediate" receiver, an action with an intermediate length bolt, but a standard length, large thread receiver ring. Such receivers are "usually" seen on the scarce, about 50 made, Rigby Mausers in .303. While the .303s have slant box magazines to feed the rimmed cartridges, the short intermediate receiver is combined with a common intermediate magazine on Corbett's rifle. I don't know how Rigby really reproduced the short intermediate receiver, but my suggestion was: Take an ordinary intermediate action, shorten the receiver ring to standard length and fit a small thread barrel. This will leave you with the same thread length like a standard length receiver, but the looks of a short intermediate one. Most likely noone will ever unscrew the barrel of the replica just to look at the thread size.