I am assuming this will be an off-hand rifle? Physics tell us that the heavier and faster the bullet is, the less it is affected by the wind. Of the two caliber choices you present I would opt for the .45. It will take less lead and powder than a .50, will produce less recoil and therefore may be able to be shot more accurately. Personally, if most of your shooting will be done at ranges less than 100 yds, I would think seriously about a .40. Less lead and powder than the .45 but plenty of oomph to carry the 100 yds. At our club my son, who shoots a .40, usually wins the 100 yd matches (a lot of the others, too) and it is usually very windy. Also consider, as you think about barrel size, a rifle that is too light is worse than one that is too heavy. A rifle that is too light for the shooter doesn't have the inertia required to be held steady.