According to the wikia, it increases overall health by 30 points.
Of the sniper rifles, the Barrett .50cal, AS50, L118A, and MSR do 98 points of base damage. These rifles all have a 1.1x damage multiplier for abdomen (lower torso). Without a ballistic vest, this results in a one-bullet kill, with a vest it takes two bullets to kill.
However, these rifles also have a 1.5x damage multiplier for the chest and head; hits to these areas are still one-bullet kills.
From experience, ballistic vests make targets much more difficult to kill with a sniper rifle.
The key to QS is to make sure before you take your shot - you are completely stationary. Too many people who try quickscoping are still moving when they take their shot and this results in about a 1:4 shot-to-kill ratio ... it's frustrating as I used to fire while still moving and even though you are lined up and it is dead on them. I was sometimes too keen to get that shot off. When you are still moving it seems as though the bullet passes straight through them, when in fact it actually hasn't gone where you intended it to go.
Secondly try to shoot from cover at first. Get into the habit of ducking and going prone, then standing - taking your shot and dropping as you shoot. This is known as a drop shot.
When you get good - you can start to drag scope, which is basically where you go to scope in and your crosshairs aren't lined up with the enemy - you have to VERY QUICKLY drag the scope right or left and time the shot and fire when crosshairs are over your opponent.
Once you have mastered the basics - you can look at 360's, stalls and no-scopes.
Remember, line up target, stop, shoot. You want to consider a sensitivity of something between 6 and 10. I play on 9, but worked up from 5. 10 i find is just a bit too erratic for anything other than a run and gun.
Really you just have to practice, there is no shortcut to no-scoping.
Hope this helps
Best Answer
No, they do not.
I believe using kill streak weapons like using the predator missile to kill another player, does count towards another kill streak, but I don't think that SAM turret aircraft kills do.