Re: load for 338 lapua
I use primarily Hodgdon H1000 and Retumbo due to the low standard deviation rate of these powders. I also work the loads up to the maximum pressure that I can safely shoot which is always, approximately, a couple of grains higher than the maximum recommended load because I have found that this cartridge, from the perspective of accuracy, likes to be loaded hot. I have found that the same is true of the Remington Ultra Magnum cartridges also.
Start with the recommended starting load and work up gradually. I shoot my brass a couple of times only because I'm hammering the hell out of the case head. It is expensive, but last weekend I shot my MK100 rifle with hot handloads and the best 5 round group shot was .524" at the widest point in the group, from damaged edge of the paper to damaged edge. If you subtract the bullet diameter of .338" from .524" the group size is .186", and this is an operational magazine fed sniper rifle with an IOR Valdada 4-28x56mm Tactical scope on top.
I have shot a large number of cartridges loaded with Vihtavuori powder, but have never reloaded with it and can be of no help in recommending a load for the 338 Lapua Magnum cartridge. Recommended starting loads with N170 and a 300 grain projectile is 79.4 grains (5.15 g), and maximum recommended is 90.4 grains (5.86 g).
Hope that helps.