Here is the technique I’m going to try .

Start with 1 bullet weight/type at a time. Chose a powder (temperature insensitive types are what I use), start with charge at least 10% below max listed, load three of four sets of 10 rounds one grain of powder at a time. For example, one set at N grains, next set N+1 grains, …, etc. For set one, put up three targets in a triangle, shoot two rounds (right / left ) in one target to get an idea where the point of impact is and to "foul" the barrel. Now, shoot a right / left combo in a right / left target arrangement, let gun cool a bit, repeat twice more, retrieve targets, clean bores. Now you should have shot 10 rounds. This method allows you to see how you barrels/loads group and there point of impact. Repeat this process for the other sets. Eventually you will find one load for each barrel that prints the closets to your point of aim. This is very time consuming, but is the best method I can think of that is scientific.