Quick question on proficiency penalties.

[Scavion]

Say I've got a rogue and he throws on a Mithril Breastplate. Does he get any sort of penalty for not being proficient in it? The armor check penalty is 0 from the -1 from being masterwork and the -3 for being mithril.

[HaraldKlak]

Scavion wrote:

Say I've got a rogue and he throws on a Mithril Breastplate. Does he get any sort of penalty for not being proficient in it? The armor check penalty is 0 from the -1 from being masterwork and the -3 for being mithril.

It is correct that such an armor would remove the need for proficiency.

However, as far as I know, in this case the masterwork ACP reduction is already counted in the mithral ACP reduction.
As such, the armor will have ACP -1.

[Lincoln Hills]

The breastplate type is Medium Armor. He still needs Medium Armor Proficiency to avoid a penalty to attack rolls equal to the armor's ACP, even though the mithral "template" then allows him to act as if he's wearing Light armor (for purposes of speed, abilities such as evasion, etc.)

And, as HaraldKlak indicated, "masterwork" and "mithral" ACP reductions don't stack. I believe the total would be a -1 ACP and - unless/until Medium Armor Proficiency is acquired - -1 to attack rolls.

[ArmouredMonk13]

Armor Expert trait= ACP. No penalties for wearing a Mithral Breastplate,

[blackbloodtroll]

Comfort enchantment will reduce the ACP by one.