What does the IMS kill streak do

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.

Even with Juggernaut players are still very vulnerable to headshots (due to the damage multiplier). Most headshots 'overkill' a player so you never really realize how much more damage they actually do.

To deal with a juggernaut: crouch behind a box, wait for them to walk by and stick a semtex to them, knife them, or (the most reliable) repeatedly shoot them in the head.