Understanding the difference between cover and concealment is easy enough; bullets can not penetrate cover while concealment just hides you from view. Interestingly enough I often find both of them misused or under utilized.
Starting with cover, the most common mistake people make is defining object as cover when in fact they are not bulletproof. In actuality, not a lot of everyday items can act as true cover. On the street, cement walls, brick walls, telephone poles (if you are skinny) cars, and possibly public mailboxes (if the assailant is using weak rounds) can be used for cover but most other objects are relegated to concealment.
In your home, several inches of books on a bookshelf will work as cover if you can get the books between you and the assailant. Furniture, mattresses, and acrylic tubs are just concealment—cast iron tubs are probably good cover depending on the cartridge but I have not seen any tests on them. Looking around my own home, I don’t see anything else besides my car that would be effective cover.
Unlike what you see on TV and in the movies, car doors don’t make good cover. While I was at Mystic’s training facility in NM, I had an opportunity to see for myself by shooting a .40S&W FMJ into a car door. The rounds easily penetrated right through! The engine block is the only sure bullet stopper.
The problem that I see with many people using concealment is twofold; 1. they treat concealment as if it is cover. 2, they don’t treat concealment as merely concealment.
A common error in using concealment is thinking that since the bad guy can’t see you, he can’t shoot you. If you’re not behind true cover, he can shoot right through your concealment! If you duck behind cover, don’t stay in one place; MOVE! If your assailant knows where you entered concealment, don’t be there when he tries to shoot there. If you come out from behind cover to shoot, come out at a different place and/or a different height each time so your attacker can’t predict your movements.
People often forget that just as their own concealment can be shot through, so can your attacker’s. Very often in force-on-force scenarios I see people ignoring shoot through opportunities. Rather than shoot through a chair where they know an attacker is hiding, they walk around it to get a visual first. If the surroundings are safe to do so, light up the chair and shoot through the door!