AC values were just pure guesses, so its not surprising that it wasn't perfect. From the initial readings on the whine-o-meter it seems the problem is more c4 and jinxed than the AC itself ?
NO! There is nothing wrong with C4.

Both jinxed in conjunction with armor class is devastating, due to large amounts of shots required to even hit them with common builds, that leads to a lot of destroyed weapons, lost ammunition, lost turns etc. etc. it's easier to deal with sneakers despite their stealth advantage and currently any armor class character is more durable than any tank possible. Common builds out there have 50%(only snipers, bursters can't hit from few hexes, only if lucky burst spread) chance to hit like in the best case scenario of 7 or 8 hexes, it's easier to keep distance with an armor class character and scout than with a sneaker.