There is no standard (other than your own). I see lots of very fine rifle makers saying 1/2-1/3 MOA on a three or five shot group. Fine, still means next to nothing. I can get a .5 MOA 3 shot group with my .30-06 A-Bolt if you give me 200 rounds and all day at the range. Anyone who's had AP or college statistics understands statistical insignificance. Even a barrel burner cartridge will give you 2000-2500 shots.
I don't consider any group under 20 holes to mean anything, and it's better to have 30. I claim several of my rifles will shoot under a minute, and are good to go for competition or long range whacking. Out of those 30 holes maybe 15-20 shots are on-top of each other. 1/4 MOA? I think not. Murphy dictates it's going to be the fliers that exit the barrel when it's at an elk @ 600 yds or you're shooting for score.
If you look at Litz's WEZ it's pretty clear how little these handful of shots @ 100yds means.
The claim isn't that at a certain time I can cherry pick a .5 moa group, it's that "My rifle shoots .5 MOA." Really? So I can put it in a bench rest on a dead calm day and shoot it out to 800 yds, and it'll land in a land on a 4" target every time? That is quite impressive. I'm certainly not saying there aren't rifles this precise, but I am saying everyone posting about it on the internet doesn't have one, and I don't care if it an AI, Surgeon, or other $5K+ built rifle.
Physics in an uncompromising bitch.
Tell me how your rifle shoots almost as good as the best, purpose-built, benchrest rifles in the hands of the best benchrest marksmen in the world.
World Benchrest Records
Last I checked the guys really into this tiny group thing posted on a different site, and a bunch of the guys on the above list post there regularly...
Their rifles are a leetle tighter than ours. They're not worried about stabilizing a bullet past 1000 yds either. They don't worry about getting dust or grit in their actions/triggers. They don't worry about cycling from a magazine, or getting wet, or hot or judging wind without flags, or really anything we deal with in the more practical shooting sports/professions...
The whole 3/4-1/2-1/3-1/4 thing is ridiculous IMHO, unless you're playing their game, which is very different. Just my $.02.