The famous PBY Catalina flying-boat used waist guns; they were mounted in so-called "blisters" extending beyond the hull. This permitted the gunners to fire them at more acute angles, and engage surface targets. The photographs below show these "blisters" and the mounting of the gun within them.

The naked gunner, Rescue at Rabaul, 1944. This young crewman of a US Navy “Dumbo” PBY rescue mission has just jumped into the water of Rabaul Harbor to rescue a badly burned Marine pilot who was shot down while bombing the Japanese-held fortress of Rabaul. Since Japanese coastal defense guns were firing at the plane while it was in the water during take-off, this brave young man, after rescuing the pilot, manned his position as machine gunner without taking time to put on his clothes. A hero photographed right after he’d completed his heroic act. Naked. (Via)

A waist gunner scans the sky for enemy aircraft through the blister window. A single .50 caliber machine gun in each blister was standard equipment on all Black Cats, U.S. Navy. (Via)

PBY Blister Gunner: 1942. Another shot of AOM Jesse Rhodes Waller and machine gun in a PBY Catalina at Corpus Christi Naval Air Base in August 1942. 4x5 Kodachrome transparency by Howard Hollem for the Office of War Information. (Via)

VP-23 crew member Harry La Brie checks the gun inside his blister window. (Via)

PBY waist gunner mounting port side gun blister. (Via)

Color picture of the blister gun position on a Navy Consolidated PBY Catalina with a .30 cal. machine gun. Photographed at Naval Air Base, Corpus Christi, Texas. (Via)

Catalina gunner mounting machine-gun in blister. (Via)

Catalina blister gunner. (Via)

Consolidated PBY5 “Catalina” view of Gunner manning waist machine gun, circa 1944. (Via)

Consolidated PBY-5 or 5A “Catalina” Patrol Bomber. Waist gunner checks the sights of the plane’s starboard waist .50 Cal. Machine Gun, at extreme depression. Photograph released 20 September 1943. (Via)

ARM3 R. H. Moore, USN, mans a .50 cal Machine Gun in a waist blister of a consolidated PBY-5 “Catalina” at Naval Air Station, Jacksonville, Florida, 10 March 1944. (Via)

This close-up of the side blister shows how .50 M2 machine guns built by Brown-Lipe-Chapin would be mounted in the aircraft.  When not in use the machine gun folded back inside and the Plexiglas blister rolled over the opening. (Via)

Crew member of a Consolidated PBY Catalina aircraft on port blister watch with machine gun. (Via)

(Via)

Waist gunner/observer blister. Consolidated PBY Catalina. In 1943 the US Navy's Catalina Patrol Wing No 10 established a naval base at Matilda Bay. Royal Australian Airforce Museum, Perth. (Via)

RAF Catalina flying-boat 'blister' turret gunner. (Via)

Consolidated PBY-5 waist gun position Black Cat 28 Sep. 1944. (Via)