Given how many decades I've been load-testing and doing sight-ins from my benchrest, I take it for granted that I can hold a rifle to its best performance almost all the time. I have bad days, of course, but not very many.
I've had some rifles which were not at all picky about what load would be very much tight-group. Others? There was some frustration in finally finding "the load".
"All day long", most likely, just means "reliably". That is, you go out, shoot a group, and it makes your heart all warm and runny. Good. You go out again and again you get that tight group. Same for a day/week/month later.
Away from competition, I don't really see why somebody would spend all day shooting group after group after group. I figure, go shoot a coyote or something.
