Why are you sighting in at 50 yards when that rifle will shoot 400-500 yards??

Could one reason it may be shooting low at 50 yards is that the bullet is still rising on it' s trajectory to the target and you are adjusting the scope too much? Where is it shooting at 100 yards? 200 yards? 300 yards?

I' d recomment you re-sight in at 100 yards (with impact 1" -1.5" high) and then see where it' s hitting at 50.