In particular what I would look at is the section on cover itself. This is from OLD p.149
I consider to be the highest priority rule when it comes to moving in and out of cover. The Blocked position goes on to say:
This means the quickstand and dropping to prone do not as far as I'm concerned constitute moving between the positions. Allowing that breaks the action economy and requires you introduce houserules for readied actions or overwatch allowing attacking in-cover characters.
I consider the rulings on cover to simply operate at a more coarse grained level of detail than the "popping up and firing" level that this thread seems to want.
If a character is "blocked" with no line of sight to someone and they make a ranged attack, firing around a corner, blind throwing a grenade, etc. they are in cover and not blocked until they take an additional move action to change that.
Dropping to prone behind cover so you are "blocked" is already handled in the rules, as being prone inflicts a -1d6 penalty to hit from ranged, and being in cover gives another -2d6, which means that attackers are already strongly disadvantaged without allowing you to move to blocked for free.
The case of running from one place of blocked line of sight to another is different because ending in a blocked state means you must make a move action on your next turn in order to make further attacks. This naturally deals with the idea of a "snap shot" by not allowing the aim action to be used, as said earlier in this thread.
I also want to take the chance to say that pinning down working with more abstract types of cover can also make sense even without "chipping away" at the cover, because it can also stand in for zeroing in your aim. After repeated shots if you know where your target is approximately (so the target is in cover and not blocked) then after repeated shots you can aim better.
When referring to how the spray attack on automatic weapons works, I would propose the folowing house rules:
- Consider obscuring cover distinct from protective cover
- Characters in obscuring cover can be targeted
- Characters in obscuring cover will have their defenses boosted by 7
Finally I'd consider the case of fully obscured characters by smoke grenades as not directly targetable, but by spray attacks they would be considered in obscuring cover, and I would also allow people trying to fire on opponents in the smoke with non-spray weapons by targeting specific squares at -2d6 cover penalty. If the player targets a square that is unoccupied (even if there are occupied squares on the path of the attack), their attack is wasted, although all they would be told is that the attack missed.