Martin,
When I develop a load I do it at the range. I start with one case and reload it repeatedly at 10 thou into the lands with increasing charges until i get a loose primer pocket. Because each time the case is fired and the brass work hardens this last charge will be considerably above "max". Drop that load 1 gr and fire in a virgin case. If you get 1 thou case head expansion, consider that your starting max with virgin brass, or adjust accordingly. Load ten or twenty rounds of this in virgin brass. Fire two rounds at 100 yd. If they don't touch, seat two of the remaining loads 5 thou deeper in their cases and see if they touch when fired. Repeat. When you find two shots touching, fire a few more and see what kind of group you get. If you don't find a load that puts two touching by the time you are 30 thou off then go back to the loaded length that formed the best two shot group and start dropping the charge three/four tenths of a grain until you get something promising. Charge and seating depth are somewhat interactive so after you find a charge that works you should check for optimum seating depth again. Keep in mind that by the time you find perfection your barrel is toast and their is such a thing as good enough to go shoot matches. Now you have been talking about long range, out to 1 K. It is not likely that you will develop a load at 100 yd that will make you smile at 1000 yd. Move out to 300 or 500 yd and start working on reducing vertical. Don't try to get this done with a head or tail wind. And of course the finest refinement of of your load should be at the distance that you are going to compete.
You won't get this done in any short time. So you can do what I did when I stated shooting Palma and just load what a bunch of the successful shooters are shooting. The load I use has been tweaked up a little to account for my slower twist barrel and what is by now a longer throat. There is no cartridge for which the accumulated target experience is greater than the 308 Win. 47 gr Varget with the Sierra 2155's that you already have in a Winchester (or 46 for Lapua) case with a BR2 seated at 2.800" to 2.820" is where you almost certainly will find very good performance. If 47 gr creates too much case head expansion on virgin cases for good case life then drop it down a grain for two or three shots till your cases are toughened up from work hardening and take it back up.
In my opinion, if your rifle won't perform with this load there is something else wrong.
Let us know what you discover.
Greg
Oh, to answer your question specifically, 2155's are not at all fussy about seating depth as long as you are off. I doubt you'll find much accuracy difference between 10 and 40 thou as long as they are all the same.
