[curryst]

I'm a little doubtful. I think your standard pebble would vaporize from the friction with the air. A pebble certainly wouldn't survive re-entry into the atmosphere, so there's an upper bound on the speed before it disintegrates. If you made a vacuum between you and the wall, it might work?

One reason to use larger projectiles is to deliver similar amounts of energy without having to fight things like that.