Generally, I aim off. It's quite a nice skill to explore.

Nod to the Wobbly-haters: if I want real precision, I tend to use a pistol with a better, adjustable, rear sight.

Gentle sight filing is good. (Patronisingly, echoing my (awful, hated) technical education teacher at school, never forget that that you can remove metal, but not put it back on.)

Have you tried different pellets? Different ammo can shoot to different places horizontally, not just vertically.