I don't really know what to tell you. If it is shooting a ragged hole at 20 yards, it should do the same on out to 30-40 and 50 yards no problem. Now, if you are not getting really tight groups at 20-25 yards, your groups will simply spread out with greater distance. The greater the distance, the more poor shooting fundamentals will reveal themselves.
Now having said all that....try another pellet brand and or weight. My S510 loves the Air Arm pellets. It will shoot JSB's fine, but not nearly with the same consistency as with AA pellets. Try the Air Arms 18 and even the 16 grains and see what happens. Your 510 should kill it at 50 yards no issue. Let us know what happens and if you figure anything out.
