shooting down downed troops...

I have been playing a campaign as of late and an encounter with a necron arc of some sort brought about a most interesting question. ** this arc has multiple weapon systems and somehow shoots them both each shooting phase.( IDK new necron)
first weapon fires and lands a hit on one of my Venoms. no big deal, except that he calls the kabalites as a target of his second weapon.
i was under the impression that once u call a target ( the transport) ALL weapons had to fire upon it. i know Tau has ways around this, but i dont see such rules on the arc.
any thoughts?

If i'm guessing correctly, you're referring to the Doomsday Arc (the new, long skimmer with a big gun underneath it), and that he claimed it could fire both modes in 1 turn, then you got cheated - badly.
A) A weapon can only be fired once pr turn (barring special rules for that specific weapon ofc), no matter how many different firing modes.
B) If it was the Doomsday Arc, are you sure he didnt first use the Doomsday Cannon to pop the tranporter, and then one of the Gauss Flayer arrays on the sides of it? because those are some of the only exceptions to the "must shoot all weapons at same target" rule - though to use the wording to first popo a transporter and then target the unit inside would earn him a deathly glare and a place in my blacklist

There are several reasons why it's not possible to do what he did.

First of all when a unit fires, it all happens at the same time, no matter what order you roll the dice in. This means there are no disembarked passengers on the table when the shots are fired.
Also, when you can split a unit's fire, you still have to declare all targets before you start firing the weapons. At that time the passengers are still in their transport and are no valid target.

I was also wondering if the gauss flayer arrays are hull mounted. If that's the case they can't even fire at the same target as the main weapon.

All guns from a tank fire simultaneusly. So when something fires that can pick separate targets he has to declare the targets first, then fire each weapon. In 3rd ed you could pop a tank with the LRC's mmelta and then hose the occupants with the bolters/assault cannon. Now that is not possible, you can pop the same tank with a mmelta, but the other guns on the tank are shooting it as well and once the unit has disembarked only another unit can now fire at it.

^^ absolutley correct, each unit declares its target and then fires all of its weapons. Keep in mind though that the necron arc thingy could pop your transport, and then say his unit of warriors could shoot your bailed out troops if they have not declared their shooting yet.

Let's not forget that it's sneaky, beardy, and if we can't do it with our Razorwings, then no-one should be able to!

There is a Necron gun that has the ability "Arc" and if you roll a 6 on a dice it also moves and hits another unit as well. It is on the Anahilation Barge if I remember right.

So if he A: shot you with the anahilation barge gun, rolled a 6, and then declared he is hitting the kabalite warriors, then he did it legally.

If he shot you with the top gun, then shot your warriors with the bottom gun, he did an illegal move.

A quick note though, it "arcs" to each unit around it, friend and foe. Also he has to roll a dice for each unit to see if hit "Arcs" towards that unit, so you might get lucky and watch him shoot his own unit

thank you for the input. ill be claiming a few more necron soon, ill kill a few for you peeps.