Grenade Mod

Grenade Mods (short for Grenade Modulators) are useful upgrades that modify the properties of a character's Protean Grenades by increasing their damage, changing their functionality, and/or adding elemental properties. They are named by their elemental property followed by their type, such as Explosive MIRV or Incendiary Rain. Each type of grenade mod is available in a range of different strengths that are roughly proportional to the level of the enemies encountered. Grenade mods are dropped by enemies, found in skag piles and chests, and can be given as mission rewards. They can also be bought as "Item of the day" at Ammo Vendors.

Contact
Manufacturer: Tediore (explosive) or Maliwan (elementals)
 * Contact grenades detonate immediately upon contact with an enemy when first thrown. If they do not impact an enemy they will explode a few seconds later. They are useful for characters oriented towards melee, since the closer a target is, the easier it is to hit them with a contact grenade.

Rubberized
Manufacturer: Pangolin
 * Rubberized grenades behave as a rubber bouncing ball when thrown, continuing to bounce along the ground or off any other surface until they explode. While these may appear useful for distant enemies, as it takes longer for a rubberized grenade to go off than most grenades, they are likely to take odd bounces and not reach them.
 * However, rubberized grenades also explode both on contact and on close proximity to enemies, which can help to counteract this, especially indoors. In cramped areas such as the rooms and corridors of Sledge's Safehouse, Rubberized Grenades are all but impossible for the enemy to avoid. Simply throw a grenade in the room, shut the door, and wait.
 * Exercise caution when throwing rubberized grenades, as they also have a chance of damaging the character if they bounce back.
 * There is a glitch in which a rubberized grenade thrown at a vending machine may get it stuck. It will twirl as though it is flying through the air, produce sparks from hitting the machine, then explode after a short time.

MIRV
Manufacturer: Torgue
 * MIRV grenades are highly effective cluster bombs and explode at a preset time after they are thrown. When MIRV grenades explode, they deploy several more grenades, which subsequently explode. MIRV grenades are highly effective against groups of closely concentrated enemies.
 * MIRV grenades can be used to cover a wide area of effect when used as an 'airburst' grenade; MIRV explode about three seconds after they are thrown, whether they hit the ground or not, so they can be thrown higher to raise the initial fragmentation and spread the cluster bombs farther.
 * More info can be found at Wikipedia's article on MIRVs.

Bouncing Betty
Manufacturer: Dahl Corporation
 * Bouncing Betty grenades are standard grenades that "jump" into the air before exploding and throwing bomblets straight out from the central blast. These are similar to MIRVs, but because they go up in the air before exploding and the bomblets explode on impact with any object, they can hit foes hiding behind objects which MIRVs cannot and can hit more mobile targets more easily than a MIRV.
 * Bouncing Betties also have bugs in that if they land in certain spots, the grenade will not explode and simply disappear. Bouncing Betties can also be caught in small nooks and crannies which will effectively eliminate their wide spread because the nearby geometry catches the explosion.
 * Note that the spread of bomblets from a Bouncing Bettie is erratic, and rarely forms a perfect circle around the original grenade.

Transfusion
Manufacturer: Anshin
 * Transfusion grenades are essentially vampiric grenades with no elemental properties. They explode into a storm of white trails that seek out enemies, damage them, then turn red and return to the thrower. When the red trails return they heal for the same amount of the damage they dealt. The explosion itself deals no damage, and the trails will travel an impressive range to seek out enemies. If you are already at full health, it is normally better to switch to a grenade mod which will deal more damage, but keep the Transfusion grenade mod until you do need it.
 * Note that transfusion mod grenades will actually heal the closest friendly unit, not just the character who threw the grenade. This is particularly noticeable when the throw is far away from the heat of battle, while team mates caught up in the midst of the fight. Scorpio Turrets are also treated as friendly units for this purpose and a Soldier can use transfusion grenades strategically to repair his turret. Nearby Claptraps are also treated as friendly units and may 'steal' away some of the restorative trails if they are near the blast.
 * If a Transfusion Grenade's blast radius is near a propane tank it will detonate the tank and heal the character, even if no enemies are present.

Sticky
Manufacturer: S&S Munitions
 * Sticky grenades do not bounce, instead sticking to whatever they impact with, be it a floor, wall, or an enemy, making it easier to aim. After a few seconds, the grenade explodes. Other grenades may bounce along and explode past the opponents.
 * Sticking a grenade to an enemy's weak point, e.g. a skag's open mouth, has a high chance to deal critical damage.

Rain
Manufacturer: Atlas Corporation
 * Rain grenades launch themselves into the air then explode, the resulting explosion rains down in a cone shape around the original grenade affecting all targets in the area. When choosing between MIRV, Bouncing Betty, and Rain, the deciding factor will normally be the damage rating each has.
 * Rain grenades have a unique effect for each element: Incendiary grenades pop up and launch a flamethrower-like blast downwards, corrosive grenades pop up and causes a waterfall of acid to rain down, shock causes thunder bolts to drop from the grenade, explosive causes smaller bombs to drop onto enemies. As with Bouncing Betties, Rain grenades will sometimes bug, vanishing rather than popping up and detonating.
 * Explosive Rain grenades are a special case: the bomblets from an Explosive Rain Grenade will NOT detonate until they hit the ground, an enemy, or any other solid object. Thus, lobbing an Explosive Rain Grenade high into the air can help to maximize its radius of effect.
 * Rain grenades can be preferable to MIRVs or Bouncing Betties in certain situations, such as when large pieces of cover prevent MIRV and Bettie bomblets from spreading out effectively. Inversely, Rain grenades should not be used indoors, as low ceilings prevent a Rain grenade from reaching the highest part of its arc, and maximizing its spread.

Longbow
Manufacturer: Hyperion
 * Longbow grenades travel at an extremely high rate to the point they are aimed at and explode a few seconds later. This makes them very accurate, as they avoid the need to arc to the target.
 * However, if they strike a wall or ceiling they will obey physics for a short time rather than exploding at the exact point of impact. They will teleport directly to the first object that would stop a normal bullet including walls, cover, and enemies themselves. Doors covering spawn points often do not count, even if they look solid. The grenade may go through the barrier and explode ineffectually. It is best to aim for the ground at the enemy's feet or at walls/objects immediately behind the enemy.
 * Enemies behind cover, or with no other objects nearby, can be difficult to damage with Longbows - the grenade must be aimed at and hit the enemy itself. Even if the grenade does contact the enemy, the grenade might still fall in front of their cover before exploding. Despite this, Longbow grenades are also excellent at flushing human opponents from cover; if a Longbow grenade is bounced off a wall behind a group of enemies, the resulting disarray can be very damaging to the enemy position even if they all successfully avoid the blast, and if nothing else they are likely to stop shooting while they scatter, easing the pressure on players.
 * When thrown from behind cover, Longbow grenades may occasionally impact invisible edges of the cover near the player's left side, so care should be taken in the presence of nearby objects on the left. Sniper rifles and other scoped weapons can be used to pinpoint the aim of a Longbow grenade.
 * In addition to direct combat, the combination of high accuracy and relatively low damage makes Longbow grenades an excellent choice for grenade jumping. Depending on the map, this can be far more useful than using them to damage enemies.
 * Longbow grenades have a possibly unlimited range; this can be seen in wide open spaces such as Skag Gully. Target a very distant target, even if they are right on the other side of the map, and throw the grenade; it will appear right next to them and can damage and kill from long distances.
 * If thrown in front of a enemy, since it takes longer to detonate than other grenades, the enemy could possibly move far enough away to take little damage, or no damage at all.

Proximity Mine
Manufacturer: Vladof
 * Proximity mines can be thrown and will wait to explode when an enemy comes near, with a lifetime of a few minutes. If a proximity mine lands near an enemy within its detection radius, it will only go off a second or two later.
 * Smarter enemies will avoid triggering the mine, allowing for the player to separate enemies from each other to make them easier to deal with, block off a chokepoint, or otherwise provide general area denial. Proximity mines cannot be triggered by other explosions or allies walking over them.
 * They can also be attached to just about any surface; and will detonate a few minutes after being set if not triggered by an enemy. Again, since smarter enemies will avoid them, this mod is less valuable as an offensive weapon. It may be useful as a defensive weapon when being chased, where the enemy will be harder to target with a standard grenade.
 * A useful exploit during the Circle of Death and Circle of Slaughter missions is the ability to throw a number of proximity mines into the arena before entering. The proximity grenades will explode in a few minutes, providing a small window of opportunity to booby-trap the arenas.

Glitches

 * Certain grenades such as MIRV, Rain, or Bouncing Betty, may disappear after hitting something instead of detonating. This is more likely to occur when the grenade leaves players' line of sight but should explode. To reduce the problem when throwing a MIRV, Bouncing Betty, or Rain grenade, either make sure it is in the line of sight from collision to explosion, or if necessary to take cover, hide behind objects that are placed rather than part of the ground mesh. This may not be enough sometimes, and it may be more effective to stand further back from behind cover looking in the grenade's direction to be able to see more of its surroundings, making the unseen terrain less likely to be cut for rendering power.
 * There is a glitch with the longbow grenade where after hitting an uneven surface (i.e. an NPC or a sloped ground) the effect of the grenade may be erroneously doubled.
 * There is a glitch with proximity mines in Craw's area. If several proximity mines are dropped next to the vending machines and the area is exited, characters returning to the vendors will trigger the mines and take heavy damage.

Trivia

 * "Bouncing Betty" was originally the nickname for a German land mine that would act as a proximity mine but instead of exploding when someone got close, the "Bouncing Bettie" would jump out of the ground and explode in mid-air.