You did have the Ring perfectly perpendicular to the bore when you took the measurement? Also at what locations was the ring when you measured the gap?
Is the gap that big in all cylinders? or just #6? if just #6, then maybe the scoring was deeper than what a 0.015" oversize bore would remove, and the machinist went larger on that one bore. However you should have been informed.
You are going to have to measure the bores and the pistons to figure what clearances exist there. I know Mercury brand Pistons are not perfectly round, but slightly oval when cold, but will be when warm and care is require regarding where measurements are taken, but if you used 3rd party pistons, that might not apply, and a 3rd party ring might have a different gap if it has a different expansion coefficient than a stock ring. Maybe you were shipped the wrong rings? Any paperwork with the pistons as to clearances and ring gaps?
If the machinist just bored all the holes too big, then you have a real problem, and so does he. I would expect he would be Anteing up for bigger pistons and rings and doing another rebore at no charge, though I would be leery of his capability/competence at that point
You had mentioned you had a prop with 22" pitch? I think that is way too much prop for your combo, a 17 would probably be the correct one. If you had been running the 22, then the reason for the piston damage is probably clear
