I don't know a thing about Biden's background or education, but I can assure you he is not stupid. He is an actor, playing a role designed to win the trust and affection of blue collar working folks. He only has to appear as intelligent as the dumbest guy on a bar stool at that little beer joint three blocks from the Ford factory.
He plays his role well and his 'get a shotgun' comment plays well with working stiffs, farm folks and even a few of us who have used a shotgun in an anti-personnel role and know what it is capable of. Do you need thirty rounds to defend your castle? Probably not. Biden is hoping nobody gives it any thought beyond that point.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence