I agree about the need to train with a shotgun and that the pattern at 15 to 20 feet is the size of a small plate. Aiming is required. Shotguns made sense in that two hands are used (facilitates accurate shooting), and they don't over penetrate like a rifle. More knowledgeable people (compared to me) recommend bird shot merely because it doesn't over penetrate.

If someone used a semi-auto shotgun, I would recommend a gas system as opposed to an inertia system. I have seen a 20 ga Benelli fail to reload when loosely held. Other people recommend pumps as they are more reliable.

However, whatever gun selected, training and repeated practice is very important.

The training needs to address also address legally when a gun should be used. I guess that in today's society, also having a great lawyer to keep one out of jail after an altercation is also important.