MK3A2 Grenade

The Hand Grenade is a weapon used in Half-Life 2. Most Overwatch Soldiers carry at least a few of these and use them to flush out and/or destroy enemies. This grenade is most likely fictional and looks more like a gas grenade, with a cylindrical shape, than a traditional fragmentation grenade, even though the designation "M83 Frag" is printed on the top. Due to it's design, this grenade probably relies on blast, rather than shrapnel, for it's effect. The weapon is a modified smoke grenade. The smoke contents are substituted for explosive shrapnel.

The hand grenade is first acquired in Half-Life 2's third chapter, Route Kanal.

Usage
These hand grenades are fairly low-bounce but due to their cylindrical shape, they will roll a significant distance if they land on a smooth surface, so take these attributes into account when throwing them. There are 3 ways in the game to throw a grenade: Pressing primary fire performs a standard overhand throw with a high arc and considerable momentum, this can be used fairly accurately for long range targets, but using this method, the grenade will usually bounce and/or roll a considerable distance once it lands. Pressing secondary fire performs a light underhand toss, this is more precise than the standard throw and can be very useful. If you're aiming towards the floor the grenade will simply drop to the ground, but if you aim up you'll toss the grenade in a small arc which is excellent for getting grenades over obstructions like walls or fences. Finally, if you crouch and press the secondary fire button, you will roll the grenade along the ground. This is an excellent way of getting the grenade under obstructions, like a chain link fence, for example. Be careful to only do this on fairly smooth surfaces, though.

You can pick up active grenades, either with your hands (by pressing E) or with the Gravity Gun and throw them away (or back at your enemies). Using the Gravity Gun as a grenade launcher has also become a popular tactic and involves dropping a grenade at your feet, quickly switching to the Gravity Gun (default hotkey G), picking up the grenade and launching it. Launching grenades from the Gravity Gun allows them to go further, faster; but because there is more power in the throw, the trajectory is slightly different, so you have to adjust your aim to account for that.

Tactics
Utilised properly, grenades are a very useful tactical tool. Besides simply killing enemies, they're ideal for flushing out entrenched enemies like snipers, for forcing soldiers out of cover, for scattering concentrated groups of enemies, for knocking over turrets that are otherwise hard to access, and even destroying out-of-reach power sources for forcefields.

Grenades are also very effective against zombies, as they generally move slowly and make no attempt to avoid the blast.

If a soldier throws a grenade at you, quickly equip the Gravity Gun, pick the grenade up and throw it back. Killing a soldier with his own grenade unlocks the PotatOwned achievement.

Zombines, the new enemy introduced in Episode One, sometimes use live grenades as clubs, unwittingly blowing themselves (and anyone close by) up in the process. It's possible to shoot the grenade out of their hand, but it's easier to use the Gravity Gun to pull it out of their hand and throw it away (or back at them). Grabbing a zombine's grenade with the gravity gun unlocks the Graverobber achievement.

Trivia

 * In a few scripted sequences in Episode Two, Combine soldiers throw your grenades back at you. As well, at one point, Gordon comes across a dumpster, with a fast zombie digging out boxes. If the player throws a grenade in, the zombie will throw it back out. If the player throws another grenade, it will blow, and the zombie will jump out in flames.
 * The Grenade itself is actually modelled from the MK3A2 Offensive Hand Grenade, a Grenade filled with TNT with the casing made of Tarred Cardboard to reduce the effect of shrapnel.