So. I built a gun dude, so obviously - been looking at gun rules. Again.
One big thing is cover rules, because it's the natural enemy of firearms.

Particularly the degrees of cover up to and including barrier rules. Below is my summary of the cover rules (p. 190) and barrier rules (p. 197-8). Maybe this should be a combat printout or something...
There seems to be 3 key components to the bonuses/penalties as result of cover.
1) Whether the defender can see the attacker.The key contradiction here is between p. 197 "hidden defender is considered unaware" as a blanket statement and p. 189 which says that the unaware rules "don't apply to defenders already engaged in combat". p.189 rules also defers to 'superior position' rules, which give +2 to the attacker if he 'has the jump' on the defender.
This largely affects whether the defender gets a reaction+intuition roll against a ranged attack - 'unaware' characters don't get the roll.
Obviously things like screen shares, sensors and other see-through-and-around-wall gadgets make you aware. My suggested compromise for the rest is that the defender should get the defense roll, if the attack is expected, even if unseen. The attacker must change some aspect of their attack - pattern, position, weapon used, etc... largely common sense stuff, in order to count the defender 'unaware'.
2) Whether the attacker can see the defender.Blind fire modifier of -6 is consistent in the rules. The combination of attacker's position, defender's position and the intervening cover/terrain makes this an easy yes/no.
Obviously, transparent materials provide barrier protection, but no concealment.
3) The degree of intervening cover between the attacker and defender (being <25%, 25-50%, 50-99%, 100%), assuming the defender took a 'take cover' simple action.Here, the only distinction necessary is between 100% cover and everything else. With 100% cover, the attacker cannot see the defender. With everything else - the defender gets the defense bonus as normal.
The take cover action (simple) should be used by the defender to change the degree of cover. So, leaning out of full cover is a simple action. Moving back into full cover can be achieved with your movement points, but no bonus is conferred.
Possible scenarios1. Defender behind 100% cover, 'Take Cover' action, attack expected: Attacker -6 to attack roll. Defender +4 to defense roll. 1/2/3/4 damage to cover for 1/3/6/10 bullets, and this damage is subtracted from damage to defender.
2. Defender behind 100% cover, No 'Take Cover' action, attack expected: Attacker -6 to attack roll. Defender unmodified. 1/2/3/4 damage to cover for 1/3/6/10 bullets, and this damage is subtracted from damage to defender.
3. Defender behind 100% cover, 'Take Cover action irrelevant, attack unexpected: Attacker -6 to attack roll. No defense roll at all. 1/2/3/4 damage to cover for 1/3/6/10 bullets, and this damage is subtracted from damage to defender.
4. Defender behind <100% cover, 'Take Cover' action, attack always expected: Attacker unmodified. Defender +4/+2 to defense roll. Barrier rules only if attack roll = defense roll.
5. Defender behind <100% cover, No 'Take Cover' action, attack always expected: Attacker unmodified. Defender unmodified. Barrier rules only if attack roll = defense roll.
6. Defender behind 100%, but partially transparent cover: as 4 and 5 above, but barrier rules apply to every attack.
7. Defender behind 100% transparent cover, 'Take Cover' action irrelevant: As 5 above, but barrier rules apply to all attacks.
Again - main distinction is 100% cover vs <100% cover. <100% is pretty clear. it's the 100% cover that's conflicting.