Apparently, Valve's Robin Walker had enough people asking if the Sniper's new Huntsman stunned the player at all. Here is his response as only someone from Valve would respond.
"No, the Huntsman doesn't stun. It pins dead/dying players."
This was just the first day of reveals for the upcoming Sniper update. We'll have more coverage as the days roll on with news on the new maps, achievements, and the other Sniper unlocks.