Lol, that's very specific.The file you're looking for will be in the Objects.rfa. WinRFA will let you extract all the files in the Objects.rfa or, for something simple like this, you can edit the con files directly with it. The files you will want to look for depend on the mod. For vanilla battlefield, in the Objects.rfa, you want to find these files:

Objects/HandWeapons/No4/Objects.con
and
Objects/HandWeapons/K98/Objects.con

These files define the weapon. One line defines the zoom factor. For the K98Sniper and No4Sniper rifles, the like looks like this:

ObjectTemplate.zoomFov 0.1

The float value at the end is the inverse of the magnification. 0.1 implies 10x magnification. You could set it to 0.01 for 100x magnification if you want. If you've unpacked the Objects.rfa, make the changes and repack it, then replace the original Objects.rfa with the modified one and you're ready to go.

Making changes to the Objects.rfa will affect online gaming. You won't be able to connect to servers with the modified Objects.rfa, so keep a backup of the original, if you do this. There is a way to make this change and still connect to online games. Open up the map that you want to alter the gun for with winRFA and add these lines to the SinglePlayerAxis.con and SinglePlayerAllied.con:

...
objectTemplate.active No4Sniper
objectTemplate.zoomFov 0.01

objectTemplate.active K98Sniper
objectTemplate.zoomFov 0.01
...

Add these lines BEFORE the rest of the single player scripts are run and repack the map. Now the sniper rifles have a zoom factor of 100 in single player games only.