Now that we’ve gotten the grenades out of the way, let’s get to the heavier stuff.
Some missions will outfit you with claymores and C4, which are total blessings when you have to defend a position. These explosives can cover one or more of your flanks, preventing you from being taken unaware.
Naturally, claymores are easier to use for this purpose than C4, as C4 you need to keep an eye on and detonate manually. Claymores do all of the work for you. If you are using C4, remember that you don’t need to keep the detonator out to set it off, you can just tap the reload button twice, no matter what you’re holding. (Note: This does not work in the PC version.)
There are advantages to C4. You can place it at an important choke point and then detonate it at the ideal time, rather than a claymore simply going off as the first soldier approaches it. You can also shoot C4 and claymores to set them off. This is important to remember because enemies can also do this, and on the harder difficulty levels, they absolutely will. It’s important to plant your explosives out of sight to prevent this from happening.
The grenade launcher and shotgun attachments are both very helpful for different reasons. The grenade launcher can take out smaller vehicles, even helicopters (if you’re a good shot), and is particularly useful against tight clusters of enemies. When you begin a mission with this attachment, you’re usually given a large amount of ammunition. Be sure to use it.
The shotgun attachment allows you to have a very versatile loadout. An assault rifle with this attachment will dominate at both close and medium ranges and you can always use a sniper rifle as a secondary, giving you the long range advantage as well. Your enemies won’t know what him them.
Article from Gamersyndrome.com