Well, I say rumor but I do believe consumption of alcohol is banned in competitive shooting because the relaxing effect gives the shooters an apparent edge. I'll source an article, if I can. 

Here you go. http://www.dailynebraskan.com/sports/alcohol-banned-from-rifle-competitions/article_208d46e3-8e6f-54b4-a2d5-98aef12ab40e.html