Looks alot like a plasma bolt hit the back of the spartans head which would be one of the few things to burn a hole that big and have blue steam comming off the armour.
Im reasonably sure the standard pistol would not make such a big hole in a spartans armour as it is safe to guess the UNSC would not make their supersoldiers so weak against small arms fire.
Infact the shot might have been a beam rifle shot as it goes through both the front and back of the helmet.