my headcanon: Riza instructs everyone in proper firearm usage and always reminds the guys to clean their guns regularly. Roy organizes a quarterly refresher course on marksmanship and gun maintenance. The team makes it a competition every time. Whoever loses has to buy drinks for the others afterwards. With all that training, everyone is fairly competent with firearms – including Roy since Riza insists he should know how to shoot accurately despite having his Flame Alchemy (so he isn’t so useless even when wet!).
basis: Everyone (with the exception of Falman, who - according to Havoc during the “fishing” mission with Barry - barely had experience on the field) was proficient with guns. In the 3rd Laboratory, Roy was able to target specific points on Lust in his attempt to disarm her, which shows that he has decent marksmanship skills.
God you say? Now this is intriguing, how much longer do you think your ‘God’ plans to wait before unleashing his fury? Just how many more thousands of lives must I take before he decides to strike me down?