Simple.  Lube your fired .303 cases and run them through a full-length .30-40 sizing die.  Done.

There are some minor differences between the two cases, but the only ones of real concern is the neck of the .30-40 is longer and the shoulder shape is slightly different than on the .303.  Running it through the die changes the shoulder shape. Can't do anything about the neck length, but it has caused me no problems.  It just allows the bullet more freespace before it enters the rifling.  Also, with the shorter neck you can't pack in quite as much powder, but my loads in this rifle are always on the very mild side anyway.

daveboy