I'd "build" a Seekins Havak in either 6 CM or 6.5 CM. Lots of combinations on action/barrel maker/etc you could do it with but I think it would be hard to beat what Seekins has put together, I'm doing this with the consideration that it would be used to hunt as well as "just" shoot the 10 shots at 500.

I'd make sure it had left side flush cups though....Seekins missed that and it irritates me with this rifle.