ok so recently ive been experimenting with ways to kill a Juggernaut on survival for modern warfare 3. Yesterday i finally found the best way after trying with every single weapon.

---

Ok in order to do this you will need to be level 44 or above on Survival

you will need

AA-12 $5,000, unlocked at level 44
Optional ~ Sleight of Hand - $5,000, unlocked at level 50

---

ok what you want to do is save up $5000 dollars and buy a AA-12 right at the beginning of the game. then wait until the Juggernaut round which can very depending on the difficulty you are on.

When the wave start look at your mini map to see where the helicopter is going that is bringing in the Juggernaut.When the Jugg comes down it will be in a cloud of smoke. hide close to the smoke but be behind cover because before you can see them they start shooting.

Once the smoke is gone sprint at the Jugg and start running in a circle around him. He shouldnt be able to turn fast enough to shoot you. As your running around him shoot him with the aa-12. It should take two clips to kill him (maybe 3 if your on insane.
Now this works on both standing Juggs and Juggs with riot shields as long as you hit the Jugg and not the riot shield.

ok it took me a while to find this so i hope it helps