PQCPTY ,
Whilst I have never handloaded for a .375 there is a fairly basic rule that your benchrest background may have caused you to overlook .
That is to seat the bullet in the case to a minimum of 1 calibre depth (or thereabouts) .It doesn't totally rule out good accuracy and anyone who thinks they need MOA from a .375 has his hand on it anyway . This will probably go against the grain but you may get some surprising results . My .243 will shoot in the 3's with the 55 Nosler waaay back off the lands .
It could be that your .375 just doesn't like those particular projectiles too . It happens all the time . Find something it likes and feed it that .