For home defense, you can’t beat the gauge. Like JEG said, low-brass field load of bird or turkey shot is your best bet - accuracy isn’t key, projectiles don’t have extreme penetration, and the weapon itself makes all the right noises, before and after triggers are squeezed. Plus the ammo is cheap and plentiful enough you can get a real good feel for your particular weapon without going broke in the process.
Hope I never have to use my 1100 Tactical for home defense, but if it absolutely came down to it, I’m hiring a lawyer and my guest is leaving in a rubber sleeping bag :tup:
Just a thought - a full-size M16 will have a higher muzzle velocity than a M4-style carbine - one reason the Americans are having trouble with firefights in Afghanistan. The locals are using AK’s at longer ranges than are expected for the M4. The M4, OTOH, is better for close-in engagements.