One of the problems with a shotgun slug, as yall can see from the "Impala Test", is penetration through the target, or through other materials on a miss.
Buckshot has a very limited range. My patrol shotgun with Fed reduced recoil loads is no more than a 15 yard gun before pellets start leaving a body target.
An AR on the other hand is a great gun at 0 or 500 yards, very little chance of overpenetration with proper ammo selection, and terminal performance is awsome.
Please don't get me wrong, I'm not saying having a scattergun in the trunk is a bad thing, but it has some limitations. I am partial to rifles, and there is certainly nothing wrong with a Winchester 94, but AR's seem to be cheap right now (bought one for $600 the other day), ammo is cheap, and modular as well.
Sorry about the ramble, just thought I'd through that out to stir the pot.
Glenn