If you are in the open carry mode, then your firearm is being proudly displayed to all the world, with the clear message that anyone who can go for it faster than you can, it's theirs.
Should someone try, your only chance is to be able to draw and fire it quickly, in less than one second.
That requires either a revolver, or else a semi-auto with a double or double-and-single action and a round chambered already, the hammer down, in condition #2, so that you do not need to futz with removing the safety, or with chambering a round using both hands, simply draw and pull the trigger, the fastest action possible.
Someone with a weapon already drawn, or who can defeat you without a weapon drawn, will beat you if they can do it in less than 1 second plus your reaction time.
Ergo, yes, you do need one in the chamber, but that's not all.
You need complete awareness of your total surroundings and anyone within 10 yards of you at all times.
You need to practice at least 15 minutes every day to get your draw and trigger pull to under 1 second. When you practice this, and you say "one thousand one" you should be finished before the "thousand."
Otherwise your gun (pistol or revolver) will belong to someone else, not to you.
Concealment is your next best defense. Together with that same less-than 1 second response. But now knowledge is power. And if they don't know where your gun is, then that gives you more time. Same requirement though, as you should be able to draw and fire it in less than 1 second.