Maybe "Guidelines" rather than rules would be better. I have a new Merkel double rifle drilling in 9.3X74R/20GA 3" that was regulated for 232 grain Norma Vulkan bullets. I have Norma's factory duplication load also (N203B ), so I can work up a load that should get close. But that won't satisfy me, of course. So if I use heavier or lighter bullets, and it doesn't shoot together or to the sights, how do I proceed? If the barrels shoot apart, do I increase velocity to get them to merge? If they cross, do I reduce velocity? Will a heavier or lighter bullet shoot higher or lower? If I change velocity does the POI strike lower or higher? I did some internet searched and didn't find anything. Are there general guidelines on this subject? And then there is the shotgun barrel and slug/buckshot loads..The possibilities are endless.... By the way, I also posted the same query on the DoubleGun Forum. Thanks. Allan

Allan,
You are on the right track as far as speeding them up to bring them together or reducing so they don't cross. But as far as shooting higher or lower in my limited experience that's a crap shoot. One rifle I owned shot very high with low velocity loads and increasing the speed didn't make it shoot any higher or lower. Another would lower the point of impact as I increased the speed to bring the bullets together.
The best two pieces of advice I can give is to start way below where you think the load should be so you can watch the bullets come together and to pick up Gram Wrights book, Shooting the British Double Rifle: A Modern Guide for Load Development and Use, 3rd Edition

+1 on Graeme Wright's book. It actually covers the issue you have raised in detail.