It's an item that most 6-shooters use to reload their guns quick than someone who reloads it by hand. Here's an example I found.

[img=https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcQQT7ThNNDt5amgASyfnN-p-ujO-f03YECZZCmFD7uugUVIE7zEgg]

Basically, you put the bullets on the re-loader, open up the 6-shooter's bullet chamber, push them all in at once, and start shooing again.