Good scope for a Stevens?

I have a Stevens bolt action in .223. It came with an adjustable scope, and at the time i thought it was a good deal, but I found out it won't hold zero. I had zeroed it at 9x, and dialed it down to 3x today because I had to use it on a coon at about 40 feet, and it was a bad kill. The rounds hit the coon everywhere but where I was aiming. $hit, I just eye-balled down the barrel for the 4th shot-which finally finished it. So this doesn't happen again, I want to put a better scope on; one I know is going to hold zero. Since the rifle and scope were only about $300, I don't want to put a ton of money in it, but I want to know it's going to hit where I aim. Anybody have any ideas?