Practice, Practice, Practice
Like the others have said, there is no substitute for a well placed shot. I've dropped a 160 lb 10 pointer with a 223. Wouldn't recommend it, but thats what I had at the time. I also had a friend hunting with me one time that took a large doe with a 30-06, using 165 gr core lokts. When the deer was hit, she didn't even flinch before throwing the flag and taking off. She dropped about 300 yds away. Upon field dressing the deer, the heart was in several pieces. I still can't explain that. I guess some of it just depends on the will to live. This deer was in the middle of a large cultivated field and was easy to see as it ran off and then dropped. However, if it was in the brush and you didn't see it drop, it could have easily been passed off as a miss if you didn't go down and check the area. The bullet apparently performed correctly, it left about a half dollar size exit hole and a large blood trail. The point I'm getting to is, just because you don't get much of a reaction from the animal and it runs off after the shot, don't necessarily pass it off as a miss. Check the area thoroughly before doing so. Use a gun you have confidence in and know what it can do. Theres no substitute for practice.