I agree, my primary being 5.56 but, as a back up that may result in me living or not. I say .45 any day over 9mm. 2 .45 govt to the chest or head of body armor is worn will do the trick. Or yes you can empty the mag of the 9mm. Either way, I want to get back to my primary as quickly as possible. So yes, a rifle round is always better.
I do not know if this is true or not, but I did read in an article how the .22 cal had killed more people other than military actions. So good aim should be and is the best thing for a knock down. Good controlled group to vitals seems to be the best answer for knock down.
Remember the Army went to 9mm to make NATO happy. The Army went to the berretta to make Italy happy. Not because of ballistics or quality of the weapon. Now the Army has gone back to .45 Govt in some Units for the secondary weapon.