I've had the same problem with my Airsporter carbine and that has the Williams style sight on the maxigrip rail, just can't get it to group (can't remember if it was shooting high or low) but kept running out of adjustment, annoying really as the rear sight cost a bloody fortune from Knibbs (no suprises there then) I've resorted to fitting a scope but boy is it accurate for a .22 tap loader with AA pellets.

Pete