Looks like short-fat 243 Win to me, and I don't care for short fat cases, but I think if I was going to try one or build one for a kid, this little HOWA with forged reciever and bolt and a hammer forged barrel, this barreled action from brownells would just be the ticket. Box magazine fed, apparently using existing mags. Send this off to McMillan or the likes for a custom fit stock.
Link: 6mm ARCLink: BrownellsPhil