have you lapped the barrel, inspected the crown<
what is these OAL before making contact with your bullets on lands?
maybe its running longer/shorter than your other 300's? too much/little jump?
every rifle can be different in what it likes
I have a few custom higher end rifles that HATE many brands of ammo and loads
BUT find the one it likes and its a 1/2 MOA gun all day
I about blew a gasket when I got it back from custom shop,after buying the one, and it shot like your's? till I found what it likes
some rifles are just more picky than others I think??
ALso,
on the OFF chance of things, can you swap rings base and scope from a known GOOD one?
never know, there, seen strange things happen before?