I agree with cmoutos....if there is a tree line just ground hunt and close up the gap a bit......but there is not a lot of difference in 150 yards and 180 yards. If you can shoot 1" groups at 150 yards I would wait until he was facing me and put the bullet right square in his chest.....he is not going to go anywhere.....just make sure you aim square into his chest and the bullet will reach the vitals for sure.....also make sure you have a good solid rest and if you don't feel comfortable wait until you do. Good Luck.