Not an expert in physics by any means, but I suspect rainfall, especially light rain fall, will not have a discernible effect on bullets at 25 yards. Much like wind will not have a discernible effect on bullets at 25 yards.Higher IQ anuses not withstanding, schlieren photography demonstrates quite clearly that raindrops deflect bullets quite a bit. Many say this is impossible, point being it was probably the reason for the groups more so than wet tables and slippery arms if the gun was shot off a rest.
To follow with a High IQ anus question of my own, can a gun fire in a vacuum? Are there enough nitrates in the powder to over come absence of atmospheric oxygen?