I am playing with several addons, including ACE, and never had this problem before. As of about a week ago, I noticed I was having a really hard time killing enemies. Then I noticed that my weapons are not actually shooting anything.
I am lag free, have tried several different maps and weapons, but no bullets ever actually leave my gun. I noticed that sometimes, they will hit the ground about 10 feet in from of my character, even though I am aiming parallel to the ground. If I engage the targets within about 5 feet, it works fine, but any more than that and it is no good.
I ran it without ACE, and it works fine. The issue seems to be with ACE.