The most accurate sporter rifles out of the box are probably Coopers, and Anschutz 54 series sporters, then Anschutz 64 series, incl. Weatherby-Anschutz 64, Vorquartsen 10-22, then a bunch of others including CZ American that are limited by lack of a match chamber, if nothing else.
That's not to say that certain rifles readily available in the US can't shoot as well, just that the average rifles aren't as good out of the box.
BTW: If, in fact, any .22LR fired a 5/8", 300 yard 5-shot group, it would probably be a world record. Frankly, I find it very hard to believe, unless it happened during a sanctioned competition. I'd even doubt a 5/8" group at 200 yards, even though I've made some 1/10" or better groups at 50 yards.
Picher