Sometimes the lower sun angle during winter seems to cause some variations in the neighborhood of 50 fps or more on some identical loads with my Pro-Chrono. Just a thought. At my range I shoot from West to East. My chronograph sometimes quits working about 2:00 pm due to the sun angle.
I have not noticed any velocity difference due to DBC.
^^^^^ This is one plausible answer, I have a Pro Chrono as well, but I didn't notice any velocity changes for the other rifles I shot that day.
I actually pulled the bullets on the remaining rounds as I was a good bit over regarding velocity with this load. When I first tested them, they were around 3080fps, this trip they were 3225!
The load in question:
180gr Partition
85.0gr Ramshot Magnum
Fed GM215M primer - Winchester case
