Despite this answer from 2011:
https://bg.battletech.com/forums/index.php?topic=3258.0, I've been repeatedly assured AMS doesn't count toward arc limits for weapons. I've checked both Tech Manual and StratOps (to include their respective errata) with no joy. Has there indeed been an update to that ruling? AMS is described as an "automated" system (and indeed, can operate even if the pilot/crew is incapacitated), so it makes sense it wouldn't count.