The lighter arrows(500-650isg gr) will work most of the time on a frontal shot but the shot has to be good, there is not a lot of room for error. I took two bulls this past year with a bow. Broad side double lung and heart...80lb Mathews Wake with 950 gr total weight grizzly sticks / 200 grain single bevel broad head 26% FOC....Set up worked perfectly, one I center punched rib on entry and split rib length ways and still got the lungs and heart.......one was at 25 yards and one was at 40yards. My buddy also harvested one and had a pass through right at 40yrds broadside with 70# prime bow at 28" and 850gr grizzly sticks 25%FOC (same single bevel broad head I was shooting)...........unless you want to limit yourself to just a frontal shot I would recommend at least 70# with 850ish gr total weight arrow and solid two blade broad head....try to get above 100KE value and as close to 90M or more value as you can, mine was 117KE and 98M. We have had several others attempt shots with much lighter arrows which did not penetrate past the broad head and just fell out, really just skin penetration.....Call and talk to Garrett at Alaska Bow Hunters Supply about Grizzly sticks and single bevel broad heads he will set you up with the right gear for compound or traditional large game bow hunting........I was very impressed with them and their products