In reply to mhb's unbelievable literary effort:

The twist-rate of my .375 barrel is in fact 12 inches, no surprise there.

The modified Greenhill Formula does however predict that the 285-grainer should be stable at that twist rate, so it would appear that mhb gets his mystery after all! I, on the other hand, will continue to load and shoot the marvellous Hornady 270gr spire-point bullet which has always delivered the utmost satisfaction in the hunting field.