If I BS attack a trooper outside their zone of control and line of fire, they get no aro? all they get is a guts check if I fail to put them in a null state? Only 2 things I know that could have helped, but please tell me more Sixth Sense level 2 could give them a aro . another model that has aro could have used alert to give that model a change face aro. I know this is the basic of the basic but I just want to be totally sure. and I did look through a couple of pages here.
Correct. If you shoot someone from outside their LOF and ZOC they generally get no ARO. However, if the trooper you are shooting at has Sixth Sense L2 (commonly from being in a Fireteam of 4+ members) they can respond as if they were facing towards the attacker (generally giving them effective LOF). If the Trooper you are shooting at does not get activated by an ARO when you BS Attack them (eg they don't have Sixth Sense), they (and any other allied trooper in their ZOC) may automatically turn to face the attacker via the Warning! rule at the end of the resolution of the order. If they take an ARM or BTS save, additionally this may force a Guts check. The Alert rule is a different rule which allows a model that has LOF to your active trooper to declare Alert, which allows any other allies (who may be facing an undesirable direction to their owner) to declare Change Facing in the same order. Note that this mean the Trooper who declares Alert doesn't get to defend themselves from an attack - it's like sacrificing yourself to try to warn your buddies.
One more thing I forgot to mention - if you attack them with a template weapon (like a flamethrower or light shotgun rather than a combi rifle ) they get to dodge (at -3 phys)
Actually, you're misleading @Moxie here, it depends. If there is no LoF because you're shooting someone in the back arc outside of their ZoC then they can't ARO at all, unless they have a Skill that states the contrary (such as Sixth Sense). However, if there's no LoF because you're shooting through a Zero-Visibility Zone, then they can ARO, provided they are facing your way. This is explained in detail in the Zero-Visibility rule. UPD: I am sure you didn't mean anything wrong, @Hachiman Taro, but given that the rules for this game often forget to mention certain important details that are explained in different unrelated rules, I thought it was important to highlight this. Apologies if I offended you in any way.
No not offended at all, there are a lot of specific exceptions to rules in Infinity, and that is one of them. It's hard to know at times how far to go in detailing exceptions when explaining basic interactions, for fear of being more confusing than helpful. I assumed the OP meant BS Attacking a Trooper who has no LOF only because you are outside its front arc. There are a number of other less straight forward possibilities of course. For example the attack could be Spec Fire or Guided (which would be covered under the clarification I gave for templates).
That is very true, I just thought you omitted a very popular tactic and decided to step in. The way rules are organised could definitely be better to help avoid such situations.
Piggy backing off of this. If I had a model get shot in the back IN zoc.Can I declare 'no Aro' to be sure I get the free spin to face from Warning! ? Or does 'idling' count as an ARO for the purposes of Warning! ? In some niche cases it might be worth it over risking a failure on Change-facing
You're not allowed to declare Idle as ARO. You can always choose not to ARO without telling your opponent why and without disclosing whether you even could ARO. Not AROing is the default, you need to actively declare your reactive orders if you wish to use them.