If one excludes the ability to receive fuel in flight, then comparison of ferry range obviously depends upon the maximum fuel load of each aircraft and their fuel burn rates.
Some years ago I was involved in a multinational AAR study. One topic to be covered was practical maximum fuel loads for future tankers. But rather than believe manufacturers' glossy brochure boasts, I set the following conditions for the departure and arrival aerodromes: Sea Level, ISA, still air, 10000 ft runway. Hardly very demanding and reasonably typical of most large European aerodromes.
The US representatives immediately asked for 12000 ft - which we refused on the grounds that neither Heathrow nor Frankfurt were typical tanker bases.
After each group had crunched the numbers, the answer was that both the A310MRTT and A330MRTT could operate with max fuel under those conditions. The 73.5T ex-ba B767-200ER proposed by TTSC for the FSTA contract was also just able do so.
But it all went rather quiet when the US team revealed their calculated figures for the KC-46 (or whatever it was called then). Because, despite uprated engines and brakes, it could only operate with around (IIRC) 84% of its max fuel even under such benign conditions.
I gather that the expensively-extended Pratica di Mare is already proving a challenge for the smaller-capacity KC-767I (which doesn't have the uprated systems of the KC-46A), so (as confirmed to me many years ago by a Boeing FSTA bidder), it is highly likely that the ability of the KC-46A to operate at MTOW will be somewhat limited in practice. Some idiot USAF fighter general once spouted runway performance figures for the KC-46A, but it was clear that he'd never heard of scheduled performance or balanced field requirements....
So I doubt very much that the KC-46A will prove in practice to have a better unrefuelled ferry range than the A330MRTT.
We also looked at a standard 4 hr time on task AAR mission scenario on an AARA situated about 60 min from the take-off and landing aerodromes, using the same 10000 ft aerodrome conditions, in order to obtain a figure for the maximum available offload assuming that the tanker would land with 1 hr to tanks dry. The result was that the A310 could offload about 53% of the A330's figure - and the KC-767 about 59%....
