Can be looted from the Lonnie's Shack home robbery. This robbery is available during Chapter 3 as a camp activity with Sean. Please refer to either the Camp or Miscellaneous sections of the guide for more information on the robbery. This is a very useful revolver variant, as its fast firing speed will make up for its slightly reduced damage. The biggest issue is the big drop in accuracy, and whether or not the extra firing speed is worth it is up to you.
Source: Suprak the Stud's Red Dead Redemption 2 Guide