have you tried to touch-up the crown ?
Dan,Had to browse Internet's specialized sites to understand what's fire lapping. Found that : http://www.ktgunsmith.com/firelapping.htm.But how should I proceed with such an air rifle ? Though my poor pumper shoots really badly, I don't want its barrell to be even more damaged, if the problem lies there.Yves
Looks like your barrel is moving, from shot to shot. Check the breech and front sight / barrel band for movement. Just for giggles, try placing a wedge between the barrel and pump tube, just behind the front sight.It's either that, or the scope is changing the point of impact due to being jarred by the pumping.Just a couple of thoughts.
Had a thought , does the pellet probe push the pellet far enough past the tranfer port? Sometimes the air blast can dammage a pellets skirt and send it wherever.