Ron is immune to the Killing Curse.
In the seventh chapter of the second book, Malfoy calls Hermione a Mudblood for the first time, and Ron tries to cast a spell at him. This spell is described as a jet of green light. Avada Kedavra is also described as green light. Therefore, Ron obviously tried to murder Malfoy (who was very lucky that Ron tried to do it with a broken wand). When it rebounded on Ron, it merely caused him to vomit slugs. Thus, Ron bears inherent magical protection from the Killing Curse (and possibly other curses as well), turning it into something much more harmless when used on him.
•...spells aside from Adava Kedavra can be green, y'know.
•Hey, it's a WMG; just go with it!
◦That doesn't protect it from being outright stupid.
•So, a twelve-year-old wizard with no apparent interest in the Dark Arts and no special talent for duelling managed to cast a silenced Killing Curse? It's a damn shame he never got a chance to duel Voldy.
•Two problems with the theory: there's a specific name for the slug-vomiting curse in question, it's called Slugulus Eructo. Second, in the movie, Ron even says outright "eat slugs!"