In reply to jharry3 :
As a guy who held certifications to instruct for both ARs and shotguns, and has spent over 25 years training both professionals and amateurs, I'm gonna disagree, because most of what you said is objectively wrong. The only time I would ever recommend a shotgun over an AR for defensive use is in a place where ARs are not legal to own and shotguns are, and even then I might suggest other alternatives.
1 - buckshot will penetrate in typical residential structures more than 5.56 especially when the 5.56 is something other than FMJ - this has been borne out by a couple of decades of LE ballistics testing. 5.56 usually stops faster than 9mm, 40 and 45 in most residential building materials because it has a greater tendency to destabilize and rapidly shed energy after hitting barriers.
2 - 12ga 00 produces about 15% more muzzle energy than a 55gr 5.56, but 20ga #3 buck produces only about 80% of 5.56 muzzle energy. The effective range of most shotguns is 10-15 yards with buckshot, which is barely adequate for home defense distances, and buckshot is incredibly inefficient ballistically. A properly zeroed AR is effective from muzzle to 200yds with a single POA for defensive purposes, making it a versatile 'one-gun' solution. Also, typical soft body armor will stop buckshot, but will be defeated by 5.56...
3 - 20ga/12ga and 5.56 all produce over 160db at the muzzle, so it's kind of irrelevant. They both suck to shoot indoors without hearing protection. Sound comparison
4 - I've done a LOT of low-light shooting with ARs and with a decent muzzle device (even an A2 birdcage) they're no worse than any shotgun I've fired under the same circumstances. In anything other than a completely dark conditions (in which you probably shouldn't be pressing the trigger), it's really irrelevant.
5 - a pump shotgun with an 18" barrel and magazine extension is only about 1-2" longer than a 16" AR, but puts a lot of weight out front as opposed to having the gun balance on the receiver. Even worse with a light mounted, and any defensive long gun needs a light. An AR is easier to handle and manipulate for most people in CQB conditions.
6 - An AR has less recoil, which makes for easier and more accurate follow-up shots, or shots on multiple targets. It's a lot easier to train with, and since the competency of the shooter is really just as important than the gun in question, this matters quite a lot. You can also get .22 conversions and .22 ARs for cheap training. Shotguns seem simple to run, but add a little stress, and they're harder to run than an AR. I've seen plenty of shooter induced malfunctions with 'dead reliable' pump shotguns.
7 - An AR is far more precise, which is always an advantage.
8 - An AR holds more rounds without reloading, and is far easier to reload when the time comes. Also easier to carry spare mags than spare shotgun ammo.
