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It is a "secret" killstreak (25, excluding strike package kills, or 24 with Hardline) that's not bound to a specific strike package.

It is similar to the Tactical Nuke in Modern Warfare 2 — using it triggers a countdown, and a bomb drops. However, unlike the Nuke, it "only" kills the entire enemy team (as opposed to all players) and disables their equipment and killstreaks for a bit.

It does not end the match, though it awards double XP for the remainder of the match for the team that used it.

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.