What exactly do you mean with "same sensitivity"? Different weapons (even ARs vs SMGs) have different FOVs when ADS, meaning it won't really be the same anyhow. If you refer to moving your mouse the same distance for 360 measurement than this is heavily discouraged and not something you would be able to achieve for all weapons, you'd have to choose.
What you realistically want is to use relative and then experiment with the monitor distance coefficient value, this value is what scales the sensitivity for different FOVs (aka different weapons when ADS). The default is 1.33 which is the same as CS GO and other games and this means, presuming you use a 16:9 monitor, you are matching 75% of your monitor distance.
I'd recommend reading this excellent thread if you want to have a better understanding of this concept, but if you want to keep it simple: If 1.33 feels too slow, go for 1.78 which will match your monitor distance 100%. If it is too fast, try 0% which is actually by a lot of people (including clever ones who have done the maths) the preferred value, and is what Apex Legends eg uses by default. If you've never used this, it will take some time getting used to and on a lower base sensitivity, sniper scopes might be difficult but for tracking with regular guns, it has improved my tracking greatly.
Otherwise, go with any arbitrary value between 0 and 1.78 as this means it will still scale all ADS FOVs with this value, whichever you feel is best.