What type of rifle is it? What finish is on the barrel? Semi-auto or bolt? What are you going to use it for? Max range?
Without knowing more details hard to give a solid recommendation. I would stick to 20" (shorter and the velocity loss and blast increase become pretty noticeable). 18" is about as short as I would ever want a .308. Find someone who is competent whether that be local or shipping it off. You also need to decide what kind of crown you want after it is shortened. As long as the job is done correctly, accuracy of the rifle should not change.
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