Last week I worked up a new load. My chronograph was giving me a reading of 2915 fps Ave. This week the same load was 2886 fps. The second shot on a clean barrel is the only shot out of 11 that went 2921 fps the rest were all 2800s. Sunny skys and only 5 degree difference in the temp from last week. I was using a Win.Large Rifle primer, I decided to try a CCI 250 mag primer. Cut the load back a .5g and it went 2827 ave. Upped the charge back up .5g. It went 2826 ave fps. It did get cloudy, could that be the reason it got slower. This is not the first time this has happened. I have never been able to get the same velocity from day to day. I have had a difference of 200 fps before. I didn't have time to go back and use a WLRP to see what the reading would be. It was also the same brass that I shot last week. It was still shooting good all shots were under MOA out to 500 yds. Impack did not change much with the mag primers. Maybe 1/2" higher at 200 yards. That does not make sence either being slower???