Actually, the bullet is supersonic, pushing a bow wave/shock wave that vaporizes/displaces water droplets enroute to the target. In my own experience; shooting on days where periodic showers occurred, the same load gives the same on target performance with or without the rain. Groups and POI don't change for me. If conditions permit, try to observe trace. In rain, it appears to be a snake of fog.
When we observe rain, the major significant component is wind. Showers and fronts behave differently. A front is preceded by gusting winds, once it passes, sustained rain tends to have steadier, lower winds. The winds from showers are not, AFAIK, predictable. Winds vary at different distances, as does air density, and the entire air mass is never at complete rest. During spells of 'calm' the major air movement vectors are vertical.
Mirage is an issue, parallax is an issue, barrel temperature is an issue, especially if the round is allowed to 'bake' in the hot chamber. For a
small investment you can
know the barrel's temperature, and not need to guess.
The performance of a load changes as the propellant temperature changes. The total energy of a powder charge includes the latent heat in the propellant. If it starts out at a lower temperature, it ends up at a lower temperature; establishing a given pressure level. With greater initial heat, more heat and pressure may result, depending on the kernel coatings/composition.
This variance can shift the load in and out of the accuracy node.
Fouling is an issue. All powders have a kernel coating, there to moderate burning rate, and more importantly to prevent static electrical discharge in the cartridge and in the powder can during transit. Since before the turn of the 20th century, that coating has been graphite. Graphite is a dry lubricant, which can vary velocity by its absence or presence. Copper fouling at the temperatures and pressures where the bullet passes in transit also possesses lubricant characteristics.
Newer coatings are more complex, but I suspect that there may be things like moly and or boron in them, which are dry lubricants. The cleanliness/fouling states are critical.
If you are starting out with a whistle-clean bore, you may not have fired enough rounds to establish a steady state fouling condition, where the fouling erodes at the same rate as its deposition. This is called bore seasoning, is a crucial segment of bore initialization, and can mess with dispersion.
I did a lot of target shooting(Turkey Shoots) with black powder/round ball, and bore condition/conditioning is even more critical in that application.
I do not strive for complete fouling removal, and do my final load development acceptance testing by starting with a fouled barrel that has already been brought up to a temperature that is similar to where it would be during a competitive course of fire. This is very similar to when hunters confirm zero/foul the bore once reaching the actual hunting locale.
This assures that I am developing a load that will be at its peak performance under conditions as close as possible to those under which it will be put to work for its intended purpose.
This type of testing is considered excessive by some, but is at the far end of the spectrum from the tests you performed; and maybe that's a factor, too.
Greg
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