Thanks for the kind words, P.O.W. but I don't really consider myself all that knowledgeable. What I do know comes from personal experience with my NEF and from the
http://groups.yahoo.com/group/nef-singleshot/ website.
Steve,
I go by the loading manual. These little rifles can be pretty picky about what they are fed. Work up a load for maximum accuracy by loading five cartridges at one powder charge, then increasing the charge by 1/2 grain and loading the next five, etc. Start at least 10% below maximum charge and do not exceed the recommended maximum. Then go out and shoot to see which loading gives the tightest group.
Be aware that the NEF HandiRifle can also be pretty temperature sensitive, i.e. the first shot from a cold barrel may be way off from the subsequent shots once the barrel has warmed up. Also the .223 seems to have trouble stabilizing some bullets. The NEF-Singleshot site above has had numerous discussions about this. It also has some accurizing tips in the files section. Perklo's "Another Fine Mess" website has a great two-part article with directions for smoothing out the trigger.
Hope this helps. -Kees-