Nice work. You have now proven to yourself that with a chronograph, the right pellet, and, a lot of patience, you can set up an unregulated rifle to provide a long string of very consistent shots. If you plot the results on a graph, you will probably find that from your starting point there is a gradual rise in velocity and a gentle slope back down to your starting velocity. If you limit the extreme spread at the highest point of the curve to about 20 fps, this string can be quite long and the poi change at 50 yds negligible. You have also confirmed an accuracy fact that a lot of folks have experienced. For round nose, diablo pellets in .177 or .22, there is often a sweet spot for accuracy at around 875 fps. You sound excited - and should be. Enjoy!