[Tutorial] How to sync your coop match 

This tutorial will fix the unsync problem between 2 players in MW3 Coop - Survival and Spec Ops

1 - Open the game -> go to Video Settings -> set "Sync every frame" to Yes. Close the game.

2 - Go to Steam\SteamApps\common\Call of Duty Modern Warfare 3\players2. Open config.cfg with notepad and set cl_maxpackets to "100". Save and close the file.

3 - Set config.cfg to "read-only".

4 - Play!