don't use the sights at all, use the top rail, it's possible the rear sight is mounted completely left or right of the bore axis. If the rear sight isn't centered over the bore you're going to see a shot go off left or right if you sight through it.
It's possible the window in the barrel is not cut exact center or the hopup unit isn't holding it exact center.. unlikely but it can happen, the other issue is that the rubber could be stretched or pinched on one side and pulling the mount off center... it's also possible the nub is off center. The goal is to center the point of contact with the BB as much as you can.
This is why W holds and flat hops or R-hop work much better, you level the point of contact and it's supposed to keep even pressure on the whole contact point vs 2 convex surfaces meeting at a point. It's like trying to balance 2 billiard balls on top of each other.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.