The M-13 Deal Breaker is a R8 Revolver ported from CS:GO. it has a capacity of 6 rounds. It costs 735 E-Cash, and can be purchased from Underground Arms.
It is called Deal Breaker in your inventory, and when you hover over it with your mouse, it will tell you how much ammunition it has loaded out of 6.
From the shape of the cartridges, it is assumed to be chambered in .45 ACP or .357 magnum.