Technically a one hole group is exactly the same diameter of the slugs it was shot with. I've seen groups that look pretty close. Benchrest shooters come the closest but I think the best five shot groups recorded are somewhere + or - 0.05" larger than the slugs diameter that they were shot with. They look to the eye like one hole groups though.

I have an old 30/30 that is a one hole shooter.

Like at a 100 yds, one shot is lucky enough to hit the target, and Lord knows where the other 3 to 10 shots went!

I keep it since it has been in the family for 40 years and I hunted with it when i was a kid, with zero luck I recall.

I take it to the range and let my son and his 10 yr old friends from our cub scout troop, use it to handload the ammo and then shoot targets with it at 10 to 20 yds.

Or in the woods at popcans full of water at 20 to 25 yds.

Beyond that, it is strictly a one hole shooter.

cheers and good 'ONE HOLE' shooting
seafire

No paper/target showing between each bullet hole. Three/five shot group, no paper/target showing equals a good group. Center to center or outside dimension of the group minus the bullet diameter will give the "group size". Just remember, 20 rounds (or more) can also produce a "one hole" group. I've shot lots of those.