While the Semtex is not seen in the Campaign, it is featured in Multiplayer and Spec Ops.
Semtex takes up the equipment slot. It is comparable to the standard grenade but with some differences. It has a set timer and cannot be cooked, though the timer is shorter than a frag grenade's fuse. At close range, the delayed explosion gives opponents time to escape. However, the timer counts down while the Semtex is in flight and will explode almost immediately after landing if thrown over long distances. It has a blinking red light and beeps softly, making it easier to spot and avoid. This makes it useful for flushing enemies that have bunkered down, as they may run from cover if a Semtex charge is tossed in proximity, and if they do not run fast enough, they will be killed or severely injured, anyway. Because the grenade sticks to the first surface it touches (except glass, which it shatters while having its trajectory remain unaffected), it will not bounce and roll past the desired target. This is very useful for accurately sticking the grenade to the Headquarters, narrow platforms or hillsides. Additionally, it can be stuck to the window sill in front of an enemy rather than actually requiring the grenade go through the window (a throw which, if missed, often means a cooked grenade comes bouncing back at the thrower.) Additionally, Semtex grenades cannot be thrown back at the original sender.
Semtex is more powerful than a Frag Grenade: it has a larger explosive radius and the highest explosive damage of any equipment in the game. It is the only grenade that can, without Danger Close, destroy the large gas tanks found in Highrise and Terminal. Unlike the frag grenade, Semtex cannot be overcooked though sticking a Semtex too close will still cause a suicide.
Less frequently some people will stick Semtex to a team-mate and have them run into a group of enemies - mainly for the "The Resourceful" challenge, in an attempt to kill at least one or two enemies. This works best in Domination or Headquarters, where larger groups of enemies will likely be trying to capture an objective. It is not recommended to try this in Hardcore modes, where friendly fire may kill teammates in the process.
The explosive power and blast radius of Semtex also make it useful on rushing classes because it can clear larger areas than frag grenades and does not ever require cooking.
Since Semtex can not be cooked to detonate sooner, it is advised to arc the throw upward if possible. This will allow the explosive to still reach its target, but detonate sooner relative to the enemies noticing. Consequently, the enemies will have less time to react and are more likely to be killed if the throw was accurate.
- If a player gets Semtex stuck on their Riot Shield, it makes no difference where it is actually positioned on the shield, it will always kill the user. The only way to survive it to swap out the shield for a weapon on the ground, or use the Blast Shield.
- "C4" is printed on the create-a-class picture of Semtex. This is because the Semtex grenade is essentially a small, timed C4 bomb.
- It is possible to survive being stuck with a semtex. If the player is stuck on the head or the arms, pushing your head "through" or into a wall and putting the semtex "through" it will result in the grenade simply disappearing instead of exploding. It is possible to get stuck on the stomach survive the explosion by going prone and putting the semtex "in" in the floor.
- It is possible to stick a Semtex to any kind of aircraft, except for the high-altitude AC-130.