Mechanic question.

[I, Lucifer]

At third level o e part of the mechanic rules says I get weapon specialization in long arms, another says I get it with all my class weapons. (Except it seems grenades)

So can someone clear out up for me? It's slightly confusing to say I get it in long arms AND all weapons my class give me proficientcy with...

[HammerJack]

The mechanic class gives you small arms and basic melee proficiency, and gives you specialization with those weapons at level 3. Grenades are proficiency only.

The exocortex feature gives you long arm proficiency at level 1 and long arm specialization at level 3.

[Peg'giz]

You never get weapon specialization with grenades, it's state in the feat description:
"You can never have specialization in grenades"

[Lethallin]

Echoing that yes, only if you pick Exocortex (or Longarm Prototype Weapon) instead of other options you get the Longarm specialization.

It states that since the base class itself does not give you Longarm proficiency, a particular aspect/choice of the class does.