Are you putting a full clip into an enemy soldier, but they aren't dying? This may be because of lag. To reduce lag there are a few things you can do.
1. Make sure you have a open NAT setting
To do this you need to open your ports so your router connects to xbox live. You can do this by clicking here.
A Moderate or Closed NAT can slow down your connection, and lower the ability to connect to people.
2. Make sure the server you connected to has a ping of 75 or lower.
To see the ping check the top right corner of the matchmaking screen. Call of Duty Modern Warfare 3 first searches for ping at 50, if it doesn't find a suitable match it increases to 75 and so on.
3. Check to see if match recording is on.
Lag decreases if you turn match recording off, because theater tracks every single thing that happens in the game and records it which can slow down your connection speed.
4. If you are using a network adapter or have the Xbox Slim, try moving it closer to your router.
5. If all else fails, try checking your connection speeds. You can do so by clicking here.
If you are at 1-3 mbps range. You may want to upgrade to a faster rate. It will cost some $$ though. Contact your Internet Service Provider(ISP) for help and rates.
Remember, it is impossible to completely eliminate lag. It is apart of anything that connects to the internet.