Title says it all
In what way does it matter?
Gunslinger's Dodge (Ex): "The gunslinger can only perform this deed while wearing medium or light armor, and while carrying no more than a light load."
Table: Armor and Shields shows a regular Breastplate as being medium armor.
If it did matter,
Mithral: "Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor."
If the reason is about proficiency, then it's medium. If it's not proficiency, it probably falls under "other limitations" for which a mithral breastplate is light armor.
In what way does it matter?
Gunslinger's Dodge (Ex): "The gunslinger can only perform this deed while wearing medium or light armor, and while carrying no more than a light load."
Table: Armor and Shields shows a regular Breastplate as being medium armor.
If it did matter,
Mithral: "Most mithral armors are one category lighter than normal for purposes of movement and other limitations. Heavy armors are treated as medium, and medium armors are treated as light, but light armors are still treated as light. This decrease does not apply to proficiency in wearing the armor."
If the reason is about proficiency, then it's medium. If it's not proficiency, it probably falls under "other limitations" for which a mithral breastplate is light armor.
I apologies I meant Nimble the dodge bonuses
I apologies I meant Nimble the dodge bonuses
As Nimble (Ex) doesn't care about proficiency, a mithral breastplate would be treated as light armor.