I heard from FX, although getting a straight answer seems near impossible. The O-ring size they are selling for the .25 Impact M3 is 6.4 X 1.3mm, the same as for the M2. Why do they show the 1.27mm size at all? Is the M3 barrel schematic marked wrong? Do they not have the 1.27 mm size and are substituting the other? I don't know the answers but I received 25 of the 6.4 X 1.3 from O-Rings and More, so I'm all set, I guess.