Define well! Mine, an old MKII, did 12mm, 5 shot group with .177 20gn H&N slugs hp @ 50m. Now going to try the .177 ZAN 24gn slugs out. Got them going @ 950fps (700mm tensioned barrel with dual tp, reg @ 150, hammer maxed out, and valve adjuster @ 4th line), but haven't gotten the time to try...