Tales of a Gun Plumber
by: Don Norrie
Back in the years when I was growing-up, nick-names for people were as common as apple pie. The nick-name could be derived from a persons physical stature (Shorty or Slim), racial origin (Scotty or Paddy), hair color (Red or Blondie), or surname such as Bell (Dinger) and Martin (Mink) and many other correlations that had to do with the personal make-up of an individual.
Names were always of interest to me, and so it came about that when I enlisted in the RCAF as a Munitions & Weapons Technician, I soon discovered that nick-names were more prevalent in the military than in the civil sector.
My first posting after completing my Munitions & Weapons course, was to 402 (FB), “City of Winnipeg” Squadron. That was in 1954, and the unit was flying Mustang fighters, and Harvard trainer aircraft.
Not long after my arrival, I heard the term “wrench benders” applied to airframe and aero engine technicians; “dot chasers” or “scope dopes” for the electronics trades, and “metal bashers” for aircraft structural technicians. These names could be easily equated to the respective trade. But when I heard the name “gun plumbers” in reference to us M&W Techs I was perplexed as there seemed to be no correlation between guns and plumbing. Initially, I paid little attention to the term, as I was more interested in the Winnipeg belles and partying than air force terminology.
Finally my curiosity got the better of me, and I asked my Sergeant if he knew how this reference to us came about. Absolutely he did! Sgt. “Tommy” Thompson was an Air Gunner during World War 2, and told me it was during that time the nick-name seemed to come into vogue. Fighter aircraft such as the Spitfire, Mustang and Hurricane, for example, required all rigidly installed machine guns to be calibrated so that the bullets converged at a specific distance in front of the aircraft.
Harmonizing the Guns
This distance depended on the type of gun being harmonized, and sometimes the pilot‘s preference. This could be done by two methods. One was to live fire the guns at a target on a range built specifically for this purpose and if no range was available, then a special chart was used and calibration carried out in the hangar or shelter. These procedures were called “harmonization”.
Following WW2 the preferred method of harmonizing Mustang aircraft was in the hangar using a harmonization chart. The first process in gun harmonization of a Mustang fighter aircraft was to place the aircraft on jacks and ensure it was in straight and level flight configuration both laterally and longitudinally. For longitudinal accuracy a plumb-bob was suspended from near the tail wheel, another under the fuselage and in line with the leading edge of the wing. Sighting from the rear of the aircraft, these two plumb-bob lines were then aligned, - by eyeball - with another line painted on the harmonization chart which was placed approximately 50 feet in front of the aircraft.
The next step was to “bore site” each gun by placing a special sighting device into the breach/barrel and aligning the gun with the respective point on the harmonization chart which was especially designated for that particular weapon. The chart had six aiming points, one for each weapon. Once all the guns were bore-sited, the pilot’s gun sight was then calibrated to its special icon on the harmonization chart. The a/c was now ready for action.
During the Cold War years at the 1 Air Division Wings in France and Germany, we bore-sighted the six .50 calibre, M3 machine guns of the F-86 Sabre at a target 1000 feet in front of the aircraft. To preclude having to align each aircraft using the “plumb-bob” method, an initial alignment was carried out on a Sabre, than three positioning rectangles for the wheels of all subsequent aircraft were painted on the range apron.
This sped up the harmonization process considerably for now the aircraft had only to be positioned on the wheel marks. It was however, necessary to place the Sabre on three jacks for longitudinal and lateral levelling and stability during the firing-in process. Each of the six guns were then individually bore-sighted and “fired-in“. First, five rounds were fired to warm up the barrel, followed by a 10 round burst to ascertain accuracy. This calibration was always carried out after a major inspection and before the squadron went on air-to-air firing exercises at Rabat, French Morocco, and later Decimomannu, Sardinia.