All aspects of the BS Attack are specified when declaring the Skill, with the exception of the firing point*. This includes allocating Burst to legal targets (in LoF etc.). *Note that due to templates being placed during Skill declaration this isn't true for Direct Template Weapons, or for Impact Template Weapons that use the teardrop template.
Long story short... It's generally better to stick with short movement skill before your attack declaration.
Hm. So that I get this right: if my trooper has a LoF to the enemy, I can declare BS - then, declare Move and move forward to get a better range (by deciding that I fire at the end of the movement)? If so, what happens from the enemy's side of things? Is his shooting range also measured to the point where my troopers ends their movement?
I see! How about the following situation: At the beginning of my turn, my trooper sees an enemy I feel he can safely shoot at (let's say, the enemy has a shotgun and my guy is out of its optimal range). So, I activate the trooper with an Order and declare a BS Attack against the shotgun guy. He AROs with a Dodge... but, at the same time, an enemy TO sniper gives me a nasty surprise by decloaking and declaring an ARO BS Attack against my trooper. Can I use the second half of my Order to declare Move toward some sort of cover? Of course, it won't prevent the TO sniper from shooting at my guy before he's behind the cover - but if the sniper misses, my guy will be safe. So - is this legal or not?
Correct. However, you could instead have declared an Idle first, or a Move first, and then when the ARO reveals a previously unknown attacker, you could made your Face to Face roll accordingly with the second half of your Order. Relying on a Sniper to miss when revealing at a moment of their choosing is a losing game.
Remember that you will get to measure the shot range ONLY at the end of the declarations, before rolling the dice. You do not do that after declaring the BS Attack...
True, but I'd like to also shoot at the shotgun guy that turn. That's my primary objective at this time, Move is just a response to the surprising arrival of the sniper. If I declared an Idle in the first half on the Order, I'd only be able to Move, Dodge *or* shoot at one of the two opponents. Yes, of course. But let's say that the shotgun guy is at such a distance that it's obvious he has little chance of hitting my trooper.
Or at both. You can split burst on your active turn, but of course only between things that are legal targets at the moment of declaration.