A belly gun is often a short-barreled, or "snub nosed" revolver, for concealed carry. It may have additional, distinctive features e.g: shortened grip frame, absent front sight, de-spurred ("bobbed") hammer, or a cut-away front half of the trigger guard. Typical modern-day calibers are .38 special, .357 magnum, .44 special, etc. In the mid-19th century Remington .41 caliber rimfire derringers were very popular.
The origin of the term is obscure, but the belly gun's modern formulation dates from the first half of the Twentieth Century, and can be attributed to a small group of men, most with military backgrounds: Colonel Rex Applegate, Major Eric A. Sykes, Lieutenant-Colonel William E. Fairbairn, pistolsmith John H. Fitzgerald, and perhaps most prominently Colonel Charles Askins. In 1939, Askins wrote, "The grandest defense gun I have ever had was a Colt .45 New Service with the barrel cut down to two inches. The hammer had been dehorned ... the trigger guard was cut entirely away in the front ... the grip was shortened ... it was a whiz for the purpose intended."
The term "belly gun" may have arisen from the practice of carrying of the weapon tucked into the waistband, close to the belly. However, others think the term instead comes from the envisioned mode of employ for these guns: firing from extremely close-range into the belly of an opponent.
Belly guns, as described above, would be most suited to use at close range, as a defensive weapon of last resort. The "Fitz Special", a revolver modified as described above, was the innovation of John H. Fitzgerald.
The term may also - if loosely - be applied to any close-quarter defensive firearm such as the S&W shrouded hammer Bodyguard and Centennial hammerless compact .38 revolvers.