Perfect Dark

All you have to do for this achievement is get a laptop gun and put it into sentry mode with where there are a few enemies. Kill 50 and the achievement is yours.

Tips:

• Sentry mode is the Laptop guns’ secondary function (will contribute to ‘Double 64’)
• Laptop sentries have 50 bullets.
• You can pick a sentry back up and re-place it.
• You start with a Laptop gun on mission 7.
• There is a secret Laptop gun on mission 1 (see secrets section)

Multiplayer Solution
In multiplayer, set-up a match where it is you against 8 peacesims and set all of the guns to laptop guns. The peacesims wont attack you unless you have a gun out and even then they just disarm you and run away.

Now you need to find an area with a laptop gun you can camp in. Just set a turret down and pick up/equip another laptop gun to lure the peace sims in. They will get mown down by your turret racking up the kills. When your turret runs out, set another one with your equipped gun and pick up/equip another.