OK, let me try to explain again - 6.5 is slightly smaller in diameter than 7 mm. If you are firing a 6.5 diameter bullet through a 7mm bore, it does not contact the grooves and lands and is "free floating" down the barrel, in which case you have very little accuracy. Now, to sleeve an original 7mm barrel to 6.5 - I think that is a very hard thing to do, and will likely not be a very competent job that will hold up.
I don't know what these 3 different people that looked at it used to judge that it is 6.5mm. Frankly, I would not have any faith in some hybrid monster unless I knew the person that assembled it, and knew of the quality of work they did.