I'm a fairly new GM in Shadowrun (did a couple of runs so far) and I still have problems in combats. I think I don't fully grasp the very basics of a combat turn... My 2 questions are below, please see if you can help. Thanks!

1. Once a player is hidden behind good cover, for example a wall near an open door, they want to quickly pop out, take a shot and hide back again. Is this 'popping out' from behind a cover considered a free action? if so - they get only one free action for one IP, so they could "pop out", then shoot but not hide back, so everybody going after them can take a shot at them. Is this correct? Or can you hide back behind cover using one of two simple actions you got (and use only one simple action for shooting)? The book says an example of free action is dropping prone, so in theory I guess popping out, or hiding, would be a free action, right? But then you only get one per IP...

2. Assuming the answer to the above is that you can somehow fit "popping out from cover, shooting and hiding back" in one 3 second combat turn, let's assume the following scenario:
Player A is in a room, behind good cover (completely hidden from view).
Player B is in the same room, also behind good cover.

The combat turn runs like this: Player B goes first (he has higher initiative) and decides to run from one cover to another cover on the other end of the room. It's in his range, so he can just about cover the distance in one combat turn. Question: can he also hide behind the new cover, as a free action? Running already takes a free action, but he can sprint in which case it's a simple action.
Now Player A goes (has lower initiative). He decides he wants to pop out from behind his cover, take a shot at the running player B in the middle of his run, and then hide back again. Can he do that?
From what I understood there's only one 3 second combat turn for all of the players, so if one of the players is running throughout the whole combat turn, then if somebody goes after him, he will see him running and can react to that, correct? Or is it like "it's my turn, I run, I hide, now it's your turn and I'm already hidden so you can't shoot me"? But then they would *not* be running throughout the whole 3 seconds, only during their (variably) small slice of time, since the whole combat turn has three seconds, so that other players also get their slices of the 3 seconds, right?

What I'm getting at here is basically this: does everybody act at once during the 3 second combat turn or does each action happen when the player declares it and then it's over before the next player declares their action? This is very important for interrupting other players' actions. I understand you can delay your action and interrupt another, slower player, but I'm not talking about delayed actions. I'm talking about the normal action flow, where everybody acts as soon as their initiative allows them to.

For the sake of simplicity I'm assuming everybody has only one IP, so as not to complicate matters further.

Many thanks!

They move in parallel (when it matters), take actions in series. All movement rates are always spread across the whole Turn, and per-IP movement takes the whole IP (again, when relevant).

Unrelated: you can always trade Simple and Complex actions for Free actions, and Sprint requires you already be running.

Dont take the 1 Free Action per turn thing too seriously. It is there to prevent exploitation (Free action to Iajutsu draw grenade, free to active, free to drop, repeat 50 times if there is no limit, etc), but it defies reason at times.

For example, what if I have a sword in one hand and a gun in the other. I want to jump onto a passing helicopter via its hanging ladder... which is 6 meters away. I need to run toward the ladder (1 free action), drop my sword (1 free action) and drop my gun (1 free action) while yelling out, 'Gotcha!' (1 free action).

This would not be allowed using the strict RAW free action rules. So try and use common sense (dropping something should always be free, no limits), and keep in mind that some things could be done simultaneously (running and shouting).

As long as you didn't Run, you'd be fine: that's only 3 Free actions. And yeah, most GMs would let you drop both, and you should have lanyards anyway… I'm not sure 'Gotcha!' counts at all, as opposed to 'communicating up to 140 characters worth'.

1. This is a game mechanic thing. You should probably enforce the "second free action can be taken as simple" rule. Thus, anyone trying to leave and regain cover in 1 IP would only have one simple action to throw around.

2. No opinion. The rules seem to suggest that player B could take a potshot at player A, saying that A was still in the open when B took the shot. On the other hand, this kind of negates the effectiveness of total cover, unless you spend all your time in total cover for every initiative pass.

You know they need to really revise the movement rules to be a per IP thing, this move per combat turn confuses the hell out of a lot of people. This may mean you have to let the street sam with wired reflexes 3 walk 5 meters per IP or run 10 meters per IP (Base 24km per hour "walking" and 48Km per hour "running". A 1 IP would walk at a speed of 6 Km per hour and run at 12 km per hour. Vehicles would have to work differently though.

As it stands in RAW, in scenario 1, that would count as firing from cover (not really taking an action, it is considered part of the shooting action). and they would take the -1 penalty to their die pool to reflect this--IIRC.

In scenario 2, by RAW a 1 IP person moves the full distance during their action. However is abstracted that when a person with multiple IP's interacts with a person with 1 IP, they can shoot/affect the action of a person before the 1 IP guy covers that distance. However if the 1 IP guy can cover that distance in the one initiative pass (Like running 8 meters or so to the next bit of cover) and the multiple IP guy has 2 or 3 IP's he'll get the cover as they cant shoot him before he gets there (assuming 1 IP guy goes first--a big assumption IMHO). A 4 IP guy would be able to shoot him if he had to go 8 meters as the 1 IP guy has only gone about 6meters when the 4 IP guy takes his action. Confused?

It confuses some people, yes. Honestly, per-IP movement has never come up in our games; we're just not minis people, I guess. If you want tactical distances, the existing rules work perfectly well. You can get per-IP movement rates with *simple* math.

Still, its hard to explain that one person can shoot while walking the first quarter of their movement, but not for the next three quarters.

I think they could hold their action for a later IP (? I never do this.), so they could shoot at any point… once.