a sub - caliber automatic pistol of the double - action breech block type incorporates a frame having barrel - mounting ribs , a handle , a barrel - receiving slide with integral breech block , a firing mechanism of a larger caliber pistol , and a magazine adapted for storing a plurality of sub - caliber cartridges . the pistol includes an interchangeable barrel terminating rearwardly in a downwardly descending lug having an integral feed ramp defining a cartridge path for the larger caliber cartridges . a feed spacer is affixed to the barrel lug for defining a sub - caliber cartridge path which is elevated with respect to the first cartridge path .