Apologies for not using the perfect English wording, I only own the books in my native language. But I think it will become clear what actions I'm talking about.
In combat, you can take aim. This is a half-action and grants you a generic +1 to hit (or precision).
This action may only be followed by further aiming actions or the attack action. You may not take another action in between, not even a free action, or you will lose the bonus.
You can also aim for a specific weak spot. This is a free action that can be taken right before shooting.
In combination that means that I can aim for the head. And I can spend time to aim better. But not at the head. If I want to aim for the head, I need to do so spontaneously, because all bonus will be lost anyway once I aim at a weak spot.
Playing a sniper for the first time in sr5, this made no sense. Obviously, once I took aim I should not be able to change it. Like aiming 5 phases at the head and then spontaneously deciding to shoot somebody in the foot. That's not realistic. But spending more actions aiming for the head should be the obvious sniper move.
Did I miss a rule that allows this?
Did I miss a game play reason why that should actually not be allowed.
We house-ruled that the aiming bonus will not be lost if you declare the specific weak spot you are aiming for before you take the aiming actions. Do you see any drawbacks or options for exploit with that ruling?