Rubberhead
The information from Heym is basically correct in general but unstated factors cause variations also. Most of my experience also comes from Heym, both directly and indirectly. During most of the 1970s I lived in Germany and had them make extra barrels for my Heym O/U combination gun (double shot barrels, 60cm long) at the factory. Later I had a local gunsmith that had been "Meister" in the hunting weapons division make a set of rifle barrels for the same gun. Most of the people that worked " side jobs" on guns either worked or had previously during the day at Heym. Since a Heym blank set of barrels was used, the barrels are for practical purposes from Heym. The barrels were regulated for the 293 gr TUG bullet and shot a little over an inch group at 100m. I had them mount a scope in claw mounts and instead of good groups, it shot patterns. I took it back for re-regulation, but it was stolen from my friends shop before it was finished. The replacement was regulated with scope mounted. Before final finishing, I took it to the Rod & Gun Club range to test it myself at 100 meters. I fired an "under" and "over" as quickly as I could get back on target. The two bullet holes were touching, so I took it back for final finishing. Mounting a scope can change the POI, different bullet weights can also change it, and waiting too long between shots, for sure will change it. These changes can happen while what you learned from Heym still remains true.
Mike