I just watched a couple of scenes on YouTube...and in one scene I'm pretty sure it was a Colt based on the grip shape. In another scene, I'm almost equally sure it was an M&P.
On the show today, if you noticed that Barney put the bullet in and then rolled the cyl so the bullet was right under the hammer. So when Andy had the prisoner fire the pistol, he pulled the trigger 5 times. Then Andy pulled the trigger after taking it away, it fired. Good action on the producer.