I have not used Retumbo in my .243 AI, but I have used IMR 7828 with mixed results. The powder gets my 105's or 107's up to speed, about 3300 fps, but the best groups for my gun were not at warp speed, but just under. I have a Win M70 in .243 AI with a 28" 1-8 twist Pac-Nor tube on the...