but with a small caliber (like a .380) handgun at close range, the bullet might pass right through soft tissue and not look much different as an exit wound.

A specialist could tell though, and that's why they need to call one in.

On edit--also, if he were shot from the front but been in a kneeling or prone position, the forensic specialist could probably determine that as well . . . which would be bad for Zimmerman's story that he felt in mortal danger.