Before we can discuss the load, we need to confirm the barrel twist rate is appropriate for heavy long range bullets.
.223 needs at least 1:8, 6mm and 6.5mm needs 1:8 and 308 needs 1:10 or possibly faster.
Next we look at high BC bullets... hopefully we find some that have a reputation for being easy to tune... High BC can cause bullets to be hard to tune for accuracy so BC alone is not the whole story.
Once you settle on a bullet that sounds promising... you need to confirm the over all length that jams the lands.
Once you know that, we need to start working up max pressures. I do this while ladder testing at 100 yards.
If the rifle is accurate I load 1 round each starting from a low powder charge and working up in 0.1 grain increments. If the rifle is not so accurate I will load 2 of each.
I shoot them at dots in order (low to high) so I can see how the impact location changes as the load changes. I pay attention to the sound and especially to how the rifle vibrates when the round goes off. If you shoot a lot, you will sense this without a thought. I good load has a dead feeling recoil... a bad load vibrates and feels like it will loosen your teeth.
If the primers get too flat, or stiff bolt lift, or pierced primer... that's where I stop shooting... any remaining loads are too hot and will be disassembled.
I then look at where the impacts moved and focus on where 3 or more impacted in the same place. To this point you may find a low velocity grouping and a high velocity grouping... those are your two best loads.
Next, start tweaking the seating depth. This requires 3 round groups... or more.
I usually find that as you move closer to the lands, the velocity spread tends to tighten up and at the same time so does accuracy... then when I get too close to the lands... accuracy starts to get worse.... That's what I have typically found.
Keep in mind that changing the seating depth also changes your chamber pressure.... so once you get the best from the above, you might refine it and do one more round of tests with just small incremental changes.
Most important is to document all your findings... shoot at 8.5x11 inch targets... write the load on the targets and put them into a binder when done... so you can refer back later... you will forget.
If in the end you cant decide between two powders that provide good results... pick the one with the faster burn rate because it burns cleaner.
