Cover and Concealment
Taking cover is often a good idea when bullets start flying, unless you want your character to die as fast as possible. But contrary to what the movies would have you believe, flipping a wooden table on its side is not going to stop four guys with AKs going full auto from filling you with holes. What it might do is keep those four guys from seeing you as you get out of the way of the dozens of 7.62 rounds zipping through at 2.5 fp/s, which would have turned your poor character into a bloody mess on the floor if you didn't. If you're in a gunfight and hide behind the engine block of an SUV, you should be safe against most things not meant to stop a tank, and unlike the sad pile of mulch that used to be a table, the engine block should keep stopping bullets long enough for you to send a few back.
Types of Cover
These are not the only types of cover: just a few examples to help determine what any object has as a cover rating. When a character 'takes cover', meaning they remain behind the object, it adds the +SDM to attacking them and the +SDM to any returning fire they make from behind it. But if that character 'peeks cover' to shoot, look around, or move, they no longer take the returning fire SDM modifier, and the SDM for attacking them is reduced to only that of the Cover itself.
Going prone No structure rating; this won't stop anything. It does make you harder to hit at range: +1 SDM attacking for every 45ft. No penalty to returning fire.
A flipped table Structure rating A. +1 SDM attacking, none to returning fire.
A sheetrock wall Structure rating A. +2 SDM attacking, +1 to returning fire.
A stack of shipping pallets Structure rating A. +1 SDM attacking, none to returning fire.
A barrel of water Structure rating B. +1 SDM attacking, none to returning fire.
A thick wood wall Structure rating B. +2 SDM attacking, +1 to returning fire.
A pile of logs Structure rating C. +2 SDM attacking, +1 to returning fire.
A piece of sheet metal Structure rating C. +1 SDM attacking, +1 to returning fire.
An armored car door Structure rating D. +2 SDM attacking, +1 to returning fire.
A bullet-resistant wall Structure rating D. +3 SDM attacking, +2 to returning fire.
An APC Structure rating E. +2 SDM attacking, -1 to returning fire.
A bunker aperture Structure rating E. +4 SDM attacking, -2 to returning fire.
Structure Ratings
A Stops up to 3 damage per attack. Health pool of 15. Won't stop most bullets, but it makes it SDM 4 to shoot at characters behind it.
B Stops up to 5 damage per attack. Health pool of 30. Will stop some bullets and makes it SDM 4 to shoot at characters behind it.
C Stops up to 25 damage per attack. Health pool of 25. Will stop most bullets for a short time and makes it SDM 6 to shoot at characters behind it.
D Stops up to 50 damage per attack. Health pool of 250. Will stop all bullets and most explosives, and makes it SDM 7 to shoot at characters behind it.
E Stops up to 150 damage per attack. Health pool of 1,500. Impenetrable to most forms of unsustained attack, and makes it SDM 7 to shoot at characters behind it.
Cover Damage
Structures take damage to their health based on the damage they stop.
An attack that exceeds a structure's stoppable damage passes through the cover and damages any objects or characters on the other side, and deals damage to the cover at its maximum stoppable damage.
A structure becomes nonfunctional and ceases to stop any damage when its health reaches 0.