I've been doing some tests on Armor Rating in MP and it looks like it's being calculated differently. I'm seeing a 1:1 damage reduction ratio. It's a little more difficult with MP to identify all the factors in play, but I've been using damage received while barrier was up to record a baseline for enemy damage and comparing that to non-barrier strikes to help determine armor penetration. Comparing barrier/nonbarrier strikes between the Lesser Shade (used for my armor rating tests) to the Venatori Zealot and Venatori Gladiator, I was seeing the Lesser Shade's ArPen roughly equivalent to the Zealots and 50% lower than the Gladiator. As both the Lesser Shade and Zealot are essentially the "base line infantry" of their respective factions, they seemed like solid ArPen baselines for future tests. It is possible that they've actually got 50% ArPen and the Gladiator has 75%, but if that's the case, it seems like there'd need to be a flat 50% ArPen rating on MP enemies.
Of course my data has the flaw of assuming that things like stat bonuses are calculated the same way as they are in SP. Since none of that information is visible in MP, it's making it a little difficult to get decent data.