The hammer adjustment nut on these doesn't have the locking grub screw to secure it like the hw100 (at least that I can see), and tends to wander.
Mine arrived shooting the 10gr jsb at 720 and then over a short time steadily decreased to around a minimum of 615 with a bell curve a peak of 670. The first symptom was initial wandering POI, then suddenly changed its favourite pellet. The way these guns and their reg systems are built, they somehow behave effectively entirely unregulated when the hammer tension is too low, bell curve and all. The only difference being the typical velocity spike as it falls off the reg. It remained deadly accurate once the plateau was reached.
Solution was a simple hammer adjustment with a tiny dab of locktite to the hammer adjuster, applied with the end of a toothpick. Been bang on 720 for 24 shots from 200-120 since, with no wake up shots or bell curve at all. It's happily shooting sub half inch at 50yds with a rest/bipod and 3-12 scope.
I bought mine from Krale, and live South Africa so sending the gun back isn't really an option for the little things.