You can't master everything at first, so choose one of the four proficiencies below.
You start in Tier 1, after taking 2 Tier 1 skills you unluck the skills in Tier 2 and so forth. Having 2 skills from Tier 5, unlocks a new proficiency.
Tier 1 skills cost 100xp, Tier 2 cost 200xp, Tier 3 cost 300xp, Tier 4 cost 400xp, Tier 5 cost 500xp.
Obtaining all skills in a Tier, gives the following bonuses to the weapongroup: Aim or MA appropriately T1: +0.3, T2: +0.5, T3: +0.7, T4: +1 and T5: +1 Dmg.