I got it from one of Little Eddie's big tickets, but it was bugged at the time (no picture, no description), so I don't know if that's the legitimate source. The special effect didn't work when I tested it, but from the text I think it can only trigger if you're using an automatic weapon (which makes sense).
The special effect appears to be evasion, though not enough data yet to say what kind of power it has.
Since you seem to unlock the ability to learn Wild Kick from the Friendly Bum by shutting down the Buried Lab, I'd imagine you can unlock this by doing that and then using an Eddie's training ticket.
Data point: blocked six damage, dealt 16 damage.
I think the defensive effect isn't based on ranged power then.
Writing for the long-neglected GOC Casefiles canon.
I think this goes here:
You slot a long clip and fire a stream of ammunition into the guard, emptying the clip in a single wild spray. He takes 13 damage.
(in a shotgun blast - quick shot - cover fire - clustered shot chain, unopposed by intended target but another guard was still punching me)
EDIT: On reflection, the hunter does have reactive defense, which can lead to ranges greater than the normal x to 2x. Quite a dramatic change in this case, but that seems more likely than other results (not having done the math).
My 18 Dec 2014 combat logs for winged hunter have odd results. My best guess is this gets hidden bonus damage when it's a actual closer (but that's weird since no message change + the defense message occurs even as solitary opener).
Excerpt from the same combat:
You spray the hunter with wild shots intended mostly as cover fire, dealing 6 ranged damage.
The hunter swoops forward and grabs you, hauling you up into the sky before letting you drop.You continue laying down cover fire.
The hunter's tail snakes out suddenly, stabbing you as you fall for 5 stealth damage.(two rounds later)
You dive and tumble like an action hero.
The hunter turns its featureless gaze towards you. A terrible cold seeps through your body, dealing 7 etheric damage.You pump silver bullets into the hunter. The bullets crackle, as though they had somehow been electrified, and deal 11 ranged damage.
The hunter dives, the wind roaring under its wings, then snaps to a stop, ripping into you for 2 stealth damage.You spray the hunter with wild shots intended mostly as cover fire, dealing 22 ranged damage.
The hunter surrounds you with its icy wings, chilling you for 4 etheric damage.
My other guess was that Silver Bullets + Mercy Killer gives a negative defense effect, but based on silver bullets discussion page we've already seen the message in PvP, so that's probably out.
In the beginning, men were mortal.