The Pentagon has confirmed a report that two Iranian SU-25 military jets fired on a U.S. drone aircraft while flying over the Persian Gulf last week. The incident happened on Nov 3. According to the Pentagon, the Iranian military jets fired continuously ... (www.thefloridanewsjournal.com) More...
Their first problem was that they fired continuously and probably ran out of ammunition in less than 30 seconds.
The second is that they are firing at a moving target from a moving platform with no way of being able to account for wind (and probably not accounting for gravity).
The third is that they fired at an aircraft in international airspace. I'm not sure why the US isn't considering it an act of war, since it is.