A bullet low in the lungs or where major arteries and heart are to bleed them and a hole in and out so it can bleed from both sides leaves a good trail Liver hit is good also. It all depends on what caliber, where hit, and bullet selection. A bullet that penetrates all the way through will give a better blood trail if it hits the vitals ect Yet you want it to do damage while going through and some figure it is better if it does not go all the way through but I want a hole both sides. I figure a good blood trail is a dead deer and I have shot deer this year with a cast bullet and got good results. Placement of course is important as if you do not put the bullet in the vitals you will not get that deer or you will be in for a long walk getting it. If you shoot cast bullets larger is better but I did kill two deer this year with a 30 cal cast in a 308 and it worked fine. Expecially after I switched to a flat tip. If you hit them good a lot of times you need no blood trail they either drop or you can see them drop a small way from where you shot them. I have had this happen a lot bow hunting and rifle hunting. Other times I had a good blood trail either method and never had to go over a 100 yards. 20 t0 30 was usually more like it. Fast high energy bullets kill with shock as well as bleeding but I have found even a cast bullet in the right place kills deer just as dead maybe not dropped on the spot but they do not go far. Jim