Raindrop size is really a statistical distribution of sizes in a particular storm. The size distribution can be determined using a disdrometer, a device measuring rain impact. A general sense of the distribution can be found using measurements of radar scatter. Once the size distribution is determined, the terminal velocity of the average raindrop can be calculated.
Terminal velocity is a function of raindrop diameter:
Drop Diameter, mm -----Terminal Velocities, ft/s (from Lenard)
0.5--------------------11.5
1.0--------------------14.4
1.5--------------------18.7
2----------------------19.4
3----------------------22.6
4----------------------25.3
5----------------------26.2
5.5--------------------26.2
6----------------------25.9
6.5--------------------30.2
Now comes the first of the REAL problems: determining the spacing between the drops and the probability that the bullet is hit by a drop. If that can be determined, then you can sum the forces of the raindrop impact with the forces associated with the movement of bullet out of the gun in a vector summation. But, as suggested in another post, a bullet moving at 1000 ft/sec (681.8 mph), is moving close to the speed of sound and therefore should have a shock wave that would displace the raindrop, perhaps causing the drop to miss the bullet altogether.
What we can see is that the largest drops move at a velocity of 3% of the velocity of the bullet (30 ft/sec/1000 ft/sec) and therefore should have little influence unless the bullet is hit by many raindrops.
The question remains: if you run to get out of the rain, will you get more or less wet than if you walk?:scratch:
Ron