Most people do without training. In another life (not literally just many years ago) I was a Marine and heavily involved in several martial arts and MMA. There's a reason that there are so many variations of the phrase "everyone has a plan until they're punched in the face". Hand to hand combat with a gun in play is a more traumatic and stressful situation that 99.9999% of people will ever have to deal with. I wouldn't give yourself too hard of a time for hypothetically freezing in a situation like that.