I have never heard of anyone expressing any concern over whether a high base 12 gauge load of number 4 bird shot (a 1 and 1/4 ounce duck load) will kill a man at twenty feet. To satisfy any doubts you may have, my advice is to buy a piece of 3/4 inch plywood, put a coat over it, take it out to the range, and shoot it at 20 feet with the duckload. I think that this will put to rest any questions that you may have. If the intruder is not instantly dead, then he will be lying on the floor, gushing, and wishing he were dead.
Best Regards,
Big Paulie