I am dealing with the same thing with my 6.5-284. Accuracy is as good as ever, but velocity has gone down in the last month. So here are my thoughts on why MY problem exists. 3 weeks ago I was getting higher velocity with everything I tried, and very low SDs as well. My powders were H4831SC and reloader 17. I am getting 70 FPS less from the RL17 and 50 less from 4831. At the time I started working with the loads the average temp was 85 degrees during the day, now it is 97, Humidity was 70% now 85, and I believe most importantly, Barometric pressure was 29.80 now it is 30.08. Also I am getting pressure issues with the reloader 17. I think the pressure problems along with decreased velocity probably are due to the temp and barometric pressure. Supposedly with RL17 it is supposed to have a more uniform pressure curve but when you add variables there is no way to avoid changing that. In what way, I don't know, but it looks like I could be having a spike in pressure to dangerously high levels that is caused by my outside conditions. The reason I think this is that a pressure spike and quicker drop would considerably decrease velocity, and when working with maximum loads as I was it would cause major pressure signs. I will test my theory in a couple months when the temps and outside air pressures stat to go back down, until then it is just a theory but I got bullets to pull. With the H4831SC all I have noticed was decreased velocity but no pressure signs, on the other hand I am working with a moderate load 3 full grains under hodgdon's listed max. Now at 68 degrees, that sounds odd, I would theorize barometric pressure and humidity, but I don't know physics, just what I have read about the burning characteristics of various powders. I know the characteristics of Hybrid 100V and RL17 are supposed to be similar but the hybrid seems to be more consistent in these conditions here than the RL17, and less consistent in lower temps.