It is possible to executeGrand Slam outside of V.A.T.S. (without the required Melee skill of 50 to perform the move) by initiating a power attack (hold attack button) while moving forward. A critically hitting Grand Slam does the most damage of any melee attack in the game, and a Grand Slam sneak attack can kill any enemy in a single hit with the right perks, except for the X-42 giant robo-scorpion and Gojira.
Old Glory can successfully strike about 1245 times from full condition before breaking.
Ulysses wields a non-playable version of Old Glory. The weapon has higher damage, but lower critical damage and a critical multiplier of 1. It also has a 10% chance of knocking enemies down (unless they have Stonewall). The weapon scales to the player character's level, gaining extra damage and critical damage at levels 25, 35, and 45.
Old Glory can be found on the body of Ulysses, if one chooses to fight him.
It appears in Ulysses' footlocker upon the completion of Lonesome Road.
Old Glory was originally meant to be a weapon in Fallout: New Vegas, then named the "Eagle Pole". It was cut from the base version of the game as a usable item, but its visual resources remained intact.
The 3 of Clubs in the Fallout: New Vegas collector's edition card deck depicts Ulysses holding what appears to be a flag pole.
It is possible to get two copies of the weapon; if the player character finishes The End with Ulysses still alive he will be sitting on a rock ledge, over looking the The Divide after one returns back from the Mojave Wasteland. Alternatively one can just kill Ulysses before completing The End/The Apocalypse, take it from Ulysses' corpse and another one will be in the footlocker.
Ulysses' dialogue in Lonesome Road indicates his eagle staff is an Old World symbol.
Behind the scenes
The name "Old Glory" is a nickname of the flag of the United States.
The idea of carrying a gold or silver pole with an eagle on top dates back to the Legions of the Roman Republic in which each legion had two "Eagles" which were never to be captured. They symbolized the courage of the members of the legion. Since Ulysses was/is a member of the Legion, he may have been an eagle bearer, showing Caesar's insistence on imitating Rome.