I did some math today on weapon durability on New Vegas and I noticed that some weapons could fire the exact same amount of rounds from full condition till breaking.
Since that happened more than once I figured that there must have been something they had in common so that could happen, and the answer was simple: It was their HP.
So I tried to find the relation of those two and the math showed: <----
WD: means weapon durability, how many times the weapon can be fired regardless of ammo usage per shot. HP: The weapon's health points, you can find it in both the weapon's page and in Fallout: New Vegas weapons's info tables. z: a variable that is either 4 or 5, the weapon will fire 4 to 5 less rounds than it's HP*5 and that could be caused by: 1)the game's inability to reduce the weapons HP below 0.8-1, or 2) something I haven't thought yet.
So it's safe to assume the is always right since any weapon will fire the exact number of rounds or just one more but not any less than that.
I have been tested to only one weapon (sawed off shotgun and it's unique) that uses more than one round each time it's shot and it was proven right.
Edit: the current form of the equation is: WD=(HP*5)*(R/WF) It is 98% accurate. WD: weapon durability, how many shots can be fired. HP: weapon's health points R: Raul. It's 1 if Raul is not companion, 2 if Raul is companion, 4 if Raul is companion and has the Full Maintenance perk. WF: bullet's wear factor, it's condition penalty or bonus.
I still have to add the Built to Destroy trait.