What is barrel droop and how do I fix it?

Barrel droop is when the barrel points slightly downward relative to the receiver where the scope is mounted, so even with the scope's elevation cranked all the way up, the gun shoots low.

It is common on break barrels because the barrel pivots on a hinge and tolerances stack up. The fixes, in order of preference, are an adjustable scope mount such as the BKL adjustable, a droop-compensating one-piece mount, Burris Signature rings with offset inserts, or carefully shimming the rear ring. Avoid maxing out the scope's internal elevation, which causes the reticle to drift.