Ask your client for money to hire an expert witness. If he doesnt have it or is indigent, ask the Court for money to hire an expert witness. If you dont ask, you dont get. What do you have to lose by asking? Answers here will get you nowhere fast.
Please, please listen to Sasparilla. The prosecutor is going to have pros at his/her disposal who will testify to nearly anything the state wants. Find someone who will take the stand. That's what this is going to take.
My own non-expert but reasonably informed opinion is that a direct shot to the head with a .223 at close range is likely to cause unmistakable permanent cavity damage that will be in excess of anything expected from a ricochet. My opinion and three bucks will buy you a cup of coffee.