What Tinker said is true - a DR should theoretically shoot both barrels parallel - so that the sights would be true at whatever range the rifle was fired at.

The distance between the individual bore axis would be the distance between the individual bore's group centres.

I think that not many DR shoot thusly - but what a wonderful thing when they do.

To find if the gun is properly regulated one must shoot it to find out. When shooting, you will find if it diverges, crosses or whatever it does. In a perfect world, it will shoot perfectly parallel.

Changing bullet weights or powder charges or even just to a different powder can have an effect on the regulation of the rifle - you are in commmand. I undrstand that changing it's recoil pattern by using a slower burning or faster burning powder can change it's regulation.

Tinker is saying that if all attempts through load changing to get the rifle to shoot the barrels properly is in vain, the barrels must be re-regulated - not to re-regulate whenever you want to change bullet weights - that would be prohibitively expensive.

Sometimes, I understand, you can change bullet weights and get the barrels to shoot properly - sometimes not and thus you are stuck with the load the gun was made to shoot- whatever it is - or - have it regulated for the lighter bullet and be stuck with that one.

One of the advantages of a single barreled rifle over a double barreled rifle, is the single's ability to shoot accurately with a multitude of bullet weights- one merely sights for it - some doubles won't allow this - period - but - you can adjust powder and/or powder charges if you know what you are doing & use appropriate powders that safely allow down or up loading, and/or other bullet weights to find a load the rifle likes.

You are in control - sometimes.