I agree, at least 600 grains or more. Though I'll probably be flamed by the light bow advocates, I also think another 10# of bow weight would be advisable. Moose are very big critters. Their hide is 3/8 to 1/2 inch thick in most places, and their ribs are big and tough. Even their hair is thick and tough. Slide one between the ribs and you're good to go. Hit a rib square on, and you may not get the penetration you need for a clean kill. And yes, I have killed a moose (66# r/d longbow and 725 grain arrow).