With the Bloody Mess perk, characters and creatures you kill will often explode into a red, gut-ridden, eyeball-strewn paste. Fun! Oh, and you'll do 5% more damage with all weapons.
”— In-game description
Due to traits being completely removed from Fallout 3, Bloody Mess was changed into a perk. Bloody Mess also appears in Fallout: New Vegas, but it was not changed back into a trait. It is not currently known whether the damage increase is applied before or after damage threshold is applied.
This perk increases the chance of dismemberment, regardless of the hit location. This can render some quests more difficult to complete, due to the necessity of precise wounds.
The quest You Gotta Shoot 'Em in the Head in Fallout 3 can be harder to complete with this perk, since head shots may not count as such if other limbs are also blown off by the shot.
The quest Three-Card Bounty in Fallout: New Vegas is somewhat harder to complete for full credit since the violent death animations may not leave a fully intact head on the marks' bodies, therefore forfeiting the bonus. It is recommended to either do the quest before taking the perk or to avoid excess damage. Aiming for the legs and using low-damage weapons for the finishing blow will usually prevent the head from being knocked off. Asking any companions to switch to melee and having them kill the bounties works just as well.
Killing multiple enemies on top of one another with this perk can make it difficult to loot them all, as it can be hard to locate a fragment belonging to a particular enemy amongst those of the others. However, it is highly useful in looting enemies which are otherwise difficult to access, such as those on catwalks, since their parts will often fall to ground level.
In Fallout: New Vegas, this perk is one of only two universal and permanent damage-increase perks, the other being Lord Death. All other damage increasing perks are focused on certain types of enemies or weapons, or only apply under certain conditions.
This perk is highly useful in the Dead Moneyadd-on, as it increases the chance that the ghost people in the Sierra Madre will be dismembered by attacks instead of just knocked unconscious, since dismemberment is the only way to kill ghost people.
If gore from dismembered victims are physically picked up, their physical weight will sometimes be extremely heavy. This is due to how the engine handles ragdoll physics, and will only happen to enemies who have their weapons "equipped." Dropped weapons are also sometimes kept equipped in the dead actor's inventory, as indicated by the interface. Certain armors may also affect gore weight.
In the German versions of Fallout 3 and Fallout: New Vegas, which are free of all blood and physical sundering (with the exception of the Ultimate Edition), there is no change in the battle and death animations. In New Vegas, however, animals (except night stalkers) can be dismembered using Bloody Mess.