But is it so ???
recent 're-read 😛' of the 5th harry potter book shows that more than one way to save yourself from the killing curse might exist.
evidence : go to the 'the one he ever feared' chapter, where dumbledore fights Lord Voldy V2.0 . There voldemort tries the avada kedavra curse a few times on harry and dumbledore both. But what happens :
1 missed curse that burns a security desk, 1 burnt phonix, 1 shattered statue, one disapparation before the curse hits dumbledore.
thus we can conclude that the AK47 curse burns flamable solid objects and shatters nonflamable solid objects.
therefore the counter for the avada kedavra curse could be :
1. a simple conjuration spell, which will introduce some basic yet solid object which stands as an obstacle against the path of the curse (ray or whatever)
2. disapparation.
that's all, protection from avada kedavra curse made easy for dummies like harry potter and ronald weasley ... and maybe neville as well.