1500 yard cartridge

Jimx1
Your load seems very mild to me but 1000 rounds is sometimes all you can get out of a barrel. Some barrels fade away slowly while others just stop shooting altogether at once.
4 inches doesn't really tell me much. Was it a shot that flipped out of a group or where all shots widely spaced like a shotgun pattern?
Bad bedding will throw a wild shot as will bad bullets.
It's hard to make a load shoot that poorly out of a good gun so as everyone has already suggested 100% disassemble the gun clean it and put it back together so everything is tight.
Been shooting 50+ years and have never used a torque wrench on a gun except on the barrel and didn't need it there either. Tight is tight.
As to your original question the 338 Lapua with a 300 grain bullet and 87-92 grains of H1000 is the easy button for up to 2000 yards.
We have been shooting 2 matches a month for over 10 years now and accuracy has always been more important than velocity in all those matches.