First define "accuracy." One example puts it as the difference between point-of-aim (POA) and point-of-impact (POI), then that is accuracy. If you mean how tight your group sizes are, then that is "repeatability." [See http://www.the-long-family.com/group_size_analysis.htm]
Do the standard 5 groups of 5 shots, and average the most extreme c-t-c distance between two shots in each group, across the 5 groups. I did this, but instead did 6 groups of 4 shots due to the 8 shot magazine (no chrono, and in very light wind - not ideal test paramaters) this past summer - T8.1/Lapco Str8tshot 0.686/Hammerhead Bangstixx muzzle with Daisy Powerline CO2 and Valken Redemption that did not roll through the barrel (don't have a ball sizer). The average c-t-c spread was 3 inches at 50 feet (benched, of course, not handheld).
Your desire to put a little or a lot of attention to details (for example, whether or not you determine the average diameter of the paintballs used in your testing) is totally up to you. Let us know what you get in the end!!!!