The way you aim needs to be instinctive otherwise you will never be accurate when it REALLY counts, any fool can sit there and aim at targets or whatever, but when youve got a split second to hit the edge of a hopper for example, this kind of practise obviously wont help you.
By 'instinctive' I mean that you will have to learn to be able to guess accurately where a ball is going to go, just by looking at your opponent. You must never use the gun to aim, you must always look at what your shooting. This means that after youve fired the first shot, youll be able to guess roughly how you need to adjust your shooting position to make the second ball more accurate. Given enough practise, you will be able to look at your target and shoot that first ball with a high degree of accuracy.
Obviously, the smaller the target, the more practise it will take to hit it first time.
The other issue is the way you hold your gun. A major skill of front players is to be able to keep tight, this seems to have brought lots of long drop-forwards on to the market which bring the gun closer to the players face. Although this does allow you to get tighter when firing, your accuracy will suffer because your field of vision is reduced (by the marker been closer) and you will be less able to hold the gun steady.
Before you start to practise your snap shot, practise holding the gun tightly is your hands (but still comfortably) and work on making sure that the way you pull the trigger does not cause the gun to move. Your whole 'firing' style/setup will greatly effect your aim, so you might want to start here and then work on narrowing that margin of error down to the first ball you fire......good luck, and practise