It sounds like you're being very careful and methodical with your load development, so I suspect you already know what your next step is. Dry neck lube another 5 rounds and see what kind of ES you get. If it matches the first 5 shots, then you'll know what the story is. If it doesn't match, then it's time for more load development - or another chronograph.
With only one chronograph, it's impossible to know if you're getting credible data or not, because you've got nothing to compare to. I run three chronographs concurrently to reduce the odds of being mislead with data from a single chronograph that might be acting up. The odds that more than one will spit out bad data on any single shot fired aren't very high. At the minimum, I can qualify the credibility of any velocity data obtained when the three chronographs aren't in close agreement.
Let us know what you conclude if you're able to figure it out.
That's a good idea! Bet you get a few looks at the range. I use the PV-21
You didn't, or I didn't catch it, mention accuracy. I would recommend shooting at least 200 yards, 300 would be better.
Seeing as you appear to be very meticulous let me recommend trying this. I wouldn't recommend it to anyone that was now very particular regarding reloading.
With that same powder, maybe a little less of a charge, and same brass and primer jam that bullet well into the lead. That is drive it so much that the bullet is actually pushed into the case by the rifling. I did this by mistake on 5 reloads. Every thing was better. Pressures were not excessive, ES dropped significantly and group size shrunk to extremely pleasing levels.
The cartridge is a 338 RUM.
Another choice is a different powder......
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!
Here's a link to a Thread with some photos of the setup, and some data collected from the setup. I've run dual chronographs in tandem for a number of years. I expanded that setup by adding an Oehler 35P last Fall. When I chronograph now, each bullet flies over 7 skyscreens.
Dry neck lube will make the difference, I gave it a try after reading some one doing it, don't quote me but I think it was Carlock with a 338Edge. I don't use it anymore and started annealing to maintain consistent bullet release.
High Fence, Low Fence, Stuck in the Fence, if I can Tag it and Eat it, it's Hunting!