Your eye will work better than any product you can buy to do it. Take the upper off, pull the BCG out and rest it on sand bags or something similar. Adjust it so the target is in the dead center of the bore. Then, without moving the upper, adjust your scope(or sights) to the center of the target. Double check to make sure both are lined up on the center, then put it back together. First shot will be on paper, adjust from there. I do it at 100 yards and am within 10" of center every time. For your first time you may want to do it at 50 or even 25 yards.
Don't waste any money on a bore sight for any rifle, this will work every time.