Armor Mastery 2 and 8

Q: Do you get still the benefits of these feats when not wearing armor?

I'd allow Armor Mastery 2 and 8 to apply when not wearing armor, on the basis that you are rolling with the blow, or something similar. This means armigers still get something out of the armor feats even when they are not in armor (which admittedly would be rare).

Armor Mastery 4 and the Armiger

Apply the benefits of the armiger's armor mastery and the armor mastery 4 feat to your total Armor Check Penalty for both armor and shield. So if you have armor mastery 4 and are carrying only a shield, you'd get the benefits of the feat. You'd also get the benefit of the feat if you were wearing chainmail, or if you had chainmail and shield -- but in this last case you would get the -2 ACP bonus only once, not once for the armor and again for the shield.

Armor Mastery 6 and Armigers

Armor Mastery 6 does stack with Armiger class abilities, but armor cannot be reduced below an effective category of 'light', so it is more use for armiger dabblers than pure armigers, since the latter eventually treat all armor as light armor anyway.

Armor Mastery and Magic Weapons

Click on the Magic Weapons link for details.