ie: If a weapon is shared between several classes, does it become available to all classes that can use it if I unlock it (with certs)? Or must I unlock the same weapon for every class? Best example I can think of is the pistol upgrade. Do I need to unlock this 5 times? Cheers.
Weapon unlocks are for all classes. If you, say, buy the TR NightHawk, your Engi, LA, and HA can all use it. If you unlock scopes or extended mags, they are still available for all classes that can use it.
Certs are unlocked for the weapon. Not the class. Meaning that if you buy a 3.4x scope for a gun that is available for the engineer and the light assault class, you will be able to use the scope with both.
If I were to buy an attachment, the foregrip for an LMG for example, could I put it on a different LMG once I unlocked that second gun? Or would I have to buy the foregrip all over again?
Attachment unlocks are unique to each weapon. Foregrips apparently aren't interchangeable in the future; different rail systems or something I'm sure.