Speaking from personal experience, a 12 gauge will drop the meanest pit bull in his tracks with proper shot placement. And I do mean immediately. Speaking from my Dad's personal experience, a .45 acp will do the same to dobermans. I know of one man who killed a pit with a well placed hit from a hornbeam staff. My sister has a friend who was a sniper in the Army, and was forced to kill a dog with the prior "neck snap" method. The Army trains it's snipers in that method, it works.

Packs would be a whole 'nuther story.