Good summary of the theories behind regulation of DR's

I have only experience of 3 different O/U rifles and so far my best results have been with open sights. This makes sense as all of them have been regulated with open sights on 60-100 meters.

Put a scope or a red dot on a double rifle and the regulation will probably change to a worse grouping, due to heating of the soldered barrels. With open sights, the front sight will "follow" the barrel movement (which varies a lot on different rifles) and keep the regulation better compared to a scope. The reticle on a scope, or a red dot, won't compensate for barrel heating and thus give a wider group after a few shots.

However- most scoped DR's with soldered barrels are possible to regulate very good with the first 2 rounds, but thereafter it might vary a lot.

My personal goal with my two existing DR's is to have a grouping of 1" at 50 meters and about 2" at 100 meters. I have managed that with the open sights, but not with my scoped rifle - the groups tend to be about the double in size.