Fun fact, the shooter seems to be wielding a Colt Detective Special, third generation.
You can identify this from
-the cylinder release being the distinct Colt design, where you have to pull back on it with your left hand to let the cylinder drop, rather than the Smith & Wesson design that allows you to push forward with your thumb.
-the iconic Colt Medallion in the grip, right above his middle fingertip. This is also how you can identify the pistol that Chibi-Usa aims at Usagi's head in her first episode.
-the ejector rod shroud under the barrel that earlier models lacked, causing the ejector rod to snag when the gun was drawn
- the rounded grip of the gun, as opposed to a wide, flat grip
>and the front sight forming a steady slope instead of a rounded blade, also allowing for a smoother draw.
I'd guess the caliber to be .38, judging by the amount of space in between the chambers.