[marfish]

Called Shot:Break Weapon
...If the modified DV exceeds half of the Structure rating for the weapon (see Weapon Structure Ratings table) corresponding to the weapon, ...

Don't seem to find it anywhere.

[Xenon]

You are correct that it seem to be missing from Firing Squad.
Odds are it will be largely be based upon standard structure ratings.
Perhaps with higher structure to heavy metal melee weapons such as claymores and lower structure for less robust high tech sniper rifles...