How does "proficiency" work in Borderlands?

Question

While playing Borderlands sometimes I receive the message:

Your proficiency with weapon <XXX> has increased to level X.

I have some questions about proficiency:

1. How many kills do you need to increase a proficiency level by 1? Or does it use another mechanism (e.g. assists, total shots)?
2. Can I see which is the level of proficiency for each weapon class?
3. Can I see where I am and what I need to reach the next level?
4. More important: which bonuses grants proficiency? Are the same for each weapon or are specific to weapon class?

Answer 1

1) From what I know the proficiency you gain is dependent on the XP you receive from the kill. If you damage an enemy with a weapon and the enemy dies you get proficiency even if something else kills it eventually. You might get more though if your weapon does the kill. So, no fixed number of kills for a level up.

2) The character tab lists your proficiencies along with your current health/shield/level/xp.

3) In the character tab the proficiencies have bars similar to your xp bar. When it's full you get another proficiency level. You don't get to see exact numbers though.

4) All proficiencies improve damage. The rest of the bonuses - I think - vary. Generally the make you also shoot faster and more accurate but for example in case of the Eridan weapons you don't get an improvement in firerate but instead an improvement in recharge rate. You can review the exact bonus by hovering your pointer over the bar. On the PC for near empty bars you have to click it to see the bonus.

Answer 2

I'm not sure how many kills/whatever else it takes. I don't recall if you can check progress to the next level or not.

There's a tab in the character sheet that reveals proficiencies. Just go to inventory or char stats and switch tabs until you see it. You should be able to mouse over each proficiency and see what the current benefits are. The area in question will show you all weapon proficiencies. I believe the bonuses are weapon type specific, not class specific. I think, and may be mistaken, some classes favor certain weapons and increase in proficiency more quickly.