Definitely COM.
The only situation I can imagine where I would *want* to "skip" shots is if a BG (who needed to be eliminated and there was no other way) was hiding behind a car (properly) and a flanking manuever is not possible. If he is crowding his cover, the only practical way to get him would be to skip the rounds off the pavement beneath the car.
This is not a likely scenario. First, most BG's don't know how to use cover effectively. You see this in IPSC matches where the shooter runs right up to cover and sticks his or her handgun out, using the cover as support. Not only does this create weapon retention problems but it increases the chance that the shooter will be hit by a ricochet off the cover. If done correctly, the shooter should be at least 6 feet behind cover to avoid ricochets.
(This is no slam against IPSC shooters, as IPSC is not designed nor intended to teach real life defensive skills.)
Secondly, most BG's don't know how to use cars as cover either. Most will use the door or other non-bullet resistant part of the car -- if I had a weapon with penetrating power, I'd shoot through the cover and get them. We here at TFL know that the only real bulletproof part of a car is the engine block and the wheels -- the rest of the car is basically thin gauge steel and can be punched through by any intermediate rifle cartridge (not sure about slugs).
Aside: I think another interesting point in the scenario is the assumption that facing multiple BGs means you have to take them all down with your shotgun. Many SHF types envision the Mongol hordes streaming over the walls of their suburban homes in a riot or other type of civil unrest situation. This doesn't hold any water for me -- rioters are interested in getting a free TV, not getting a well placed blast with a 12 bore. Once you take out the first few rioters or (preferably) the leader/inciter of the riot, the Horde will go elsewhere to find that shiny 36" Sony WEGA.
Those with LE riot experience will know that riot control is not about suppressing the whole mob (attrition theory) but instead about putting the "leaders"/"inciters" out of commission (king theory). To destroy your enemy, it is preferable to attack his center of gravity instead of concentrating on eliminating his entire force.