Re: 5 or 3 shot group when determining ES/SD???
unless you don't have access to a range that's at least 600 yards, I wouldn't worry with chronograph numbers.
the target is, and always will be, the final arbiter.
chronographs are not perfect, and they'll throw the occasional lulu at you... and even loads with very tight numbers will not always group well, as most of us know.
as to groups... if you're dealing with an overbore cartridge that's hard on barrels, shoot 3 shots. if they land tight, call that group good. then move out farther, for successive groups until you've reached the longest distance you plan to shoot. perhaps you've heard the old mantra--to which there is a lot of truth--3 shots tests the rifle and load, 5 shots test the shooter. obviously there can be exceptions to this...
...but you don't want to be one of these guys that burns the throat out of their barrel shooting aggregates to impress the BR glitterati and then when you finally find a useful, real world purpose for the rifle its barrel is near shot out. bad juju death by hum-foo-foo... :o