Even at 300, wind can play a factor (as attested to at the last competition at Mission that I was able to watch a friend compete in for the first time). At 600, wind can play a major factor, so it is wise to stay in the 1-300 range (it's the lesser of two evils, 338LM bullets might take more distance to fully stabilize, BUT wind is wind and the greater evil) Hope your loading to ogive and testing by varying bullet seating depth? It's the best way to find most accurate load, with minimal shots. Once you know how well it groups, move on to far greater ranges
Even if a rifle shoots 1/4 or 1/2 MOA at 100-300, it won't at 1000 due to wind
unless you're an amazing wind reader... [OK there are other factors too - temp, pressure, coriolis effect...and most importantly...shooter skill]
You should be load developing in zero wind .... Not my first day at 1000 yards