10 shot strings for 4.0/N320 low of 915 to high of 1056. AVG: 1035, SD of 13.3, ES of 33
10 shot strings for 4.1/N320 low of 970 to high of 1058 AVG: 1019, SD of 31, ES of 86
Your first obvious problem is that the numbers you reported are not correct:
915 to 1056 is an ES of 141, not 33, and your SD should be MUCH higher than 13.3
970 to 1058 is an ES of 88, not 86
So there's that.
This is oversimplified (because I lack the knowledge to talk about this in anything more than simple terms
), but basically, how it works, the sensors "observe" the space overhead, and when something (like a bullet) disrupts the space over the front sensor, it starts the timer, and when that object disrupts the space over the back sensor, it stops the timer, and uses the distance between the two sensors and the time it took to cross that distance to calculate feet per second. Pretty basic, but because the distance between the two sensors is short, minor differences in how/when the sensor is triggered can produce significant differences in the calculated velocity.
SO... if for example, some light reflected from the nose of the bullet triggers the a sensor before the bullet is overhead, the sensor is effectively seeing the bullet early, and precision declines significantly. So light matters a lot.
With a chrono, diffused light is better than direct light. Cloudy days are perfect, so long as there's still enough light for it to function, but if there's not enough light, you'll just get failures to record shots, so you would know.
I don't use the sky screens at all. If it's an overcast day, they're unnecessary, and if it's a clear day, I set up a target stand in an adjacent position to cast a shadow across the entire chrono. This negates the issues of direct sunlight, which are supposed to be cured by the skyscreens, which I have personally found to be only marginally effective. I have a buddy with the same chrono as I have, and after I explained to him the why of what I do, he went out and bought a picnic/beach canopy, like a tent without walls, and he just erects that over the chrono every time he uses it, and when he told me what he had done, he was very excited at how much his numbers had tightened up. Pretty smart. I've thought about building a doghouse to mount my chrono in and shoot "through" but that canopy is probably just as effective.
To combat direct sunlight, some people will color the tips of the bullets to reduce light coming off the nose.
So... I use a ProChrono Digital, which uses the same basic type of optical sensors your Caldwell does (who knows, possibly identical, sourced from the same supplier -- you never know). I have found that the more reflective the surface of the bullet, the less precise the chrono readings are. To that point, jacketed is more reflective than lead or coated lead, plated is more reflective than jacketed, and XTreme is the most reflective plated bullet I've used, as shiny as a brand new penny.
Angle of light has an impact. I can't guarantee that my tricks and techniques will translate exactly to anyone else unless they're shooting at the same latitude I am with the ranges oriented in the same direction. I would imagine if I were doing this somewhere else in the country, or in the same spot but facing a different direction, that some of my techniques in chrono use would become more or less effective.
You might also want to weigh charges individually to make sure your problem isn't in the powder drop.
Do you leave powder in the hopper between loadings? That increases drop to drop variation. Always empty the drop after a loading session and start fresh each session.
And wipe down your drop with an anti-static sheet.
AND STOP using plated bullets. They're not best at anything.