Good advice Fuhrmann. To which I would add, re-crowning the barrel can help if 2 barrels won't regulate, (worked for my Bock-Drilling. A jewellers lupe can give you a feeling for the state of the crown). Also care in sandbagging, (same point of contact each time), lots of time between shots for the barrel/s to cool as thin barrels do shift their POI. Establishing what the original load was, either from proof marks and/or from old catalogues of the same era.

A change in bullet weight or type, (or powder) may help. Another thing that can help is to mark the bullet number using a spotting scope on a sheet of paper at hand. What you're trying to do is pick up if they are crossing over mid-way. A chronograph will let you track how more or less powder is working to shift POI. In short, careful observation and good records.

Sometimes Drillings were made to use lighter or heavier bullets than you would expect and suddenly, everything is meeting up.

Incidentally, I had a Drilling in 7x57R with a rail scope that needed replacing. The gunsmith "regulated" it by bore-sighting it on a house several miles away over a lake. When I shot it later at 100 yards, it was nearly spot on.