It made life interesting.
This is what I found on the 50 Vickers:-
Cartridge 12.7 x 81 mm
Calibre 0.5 inches (12.7 mm)
Rate of fire 500-600 rounds per minute
Muzzle velocity 2,540 feet per second (770 m/s)
Maximum firing range Altitude : 9,500 feet (2,900 m)
Range: 4,265 yards (3,900 m)
Feed system belt
The origin of the .5 inch Vickers goes back to the First World War. It seems that two or three different problems prompted the development of this gun. One was the need to fire bullets large enough to carry a useful incendiary charge to set light to enemy balloons and airships. Some of the rifle-calibre Vickers had been chambered for obsolete rifle cartridges such as the 11 mm Gras in order to achieve this, but a purpose-designed modern cartridge would obviously be better. The second was the need for a more hard-hitting machine gun against aircraft, some of which were now being fitted with armour. The third was the development of the tank, which required a better armour-piercing performance than rifle-calibre guns could achieve.
Although the German 13 mm Mauser M1918 anti-tank rifle and the associated TuF (Tank und Flieger, after the intended targets) heavy machine gun are now famous, the parallel British developments are not. These were based around a .600/.500 inch cartridge originally derived from an elephant gun round. The anti-tank rifle was the Godsal of 1918, the machine gun was a Vickers. The Godsal disappeared from the scene (although an example has survived) but the cartridge for the machine gun went through various evolutions (included a version with a belted case) until the final form emerged in 1921. This used a rimless 12.7x81 case and was known by Vickers as the .5V/580, after the bullet weight in grains (37.5 grams). Muzzle velocity was around 750 m/s and total cartridge weight 83 g. The ammunition was officially adopted for service in 1924.
Several different loadings of the .5V/580 round were developed for British service, as follows:
Ball Mark I.z: 580 grain bullet with two-piece core (front aluminium, rear lead). Approved for service 1924, but not issued.
Ball Mark II.z: 580 grain bullet as above, muzzle velocity 2,540 fps (774 m/s). Approved for service 1925.
Ball Mark II (cordite propellant): approved for land service 1933.
Armour Piercing W. Mark 1.z: bullet with hardened steel core and lead sleeve and tip filler. Approved for service 1925. To pass proof, seven out of ten bullets had to penetrate 18 mm of armour plate at 100 yards range at 0 (90) degrees, and 70% also had to penetrate 14 mm armour striking at 20 (70) degrees.
Armour Piercing W. Mark 1 (cordite propellant): as above, approved for land service 1933.
Semi Armour Piercing F Mark 1.z: bullet as AP except steel core not hardened. 2,470 fps (753 m/s). Approved for naval use 1938. To pass proof, 70% of bullets had to penetrate 15 mm armour plate at 100 yards and 0 (90) degrees.
SAP Tracer FG Mark 1.z, II.z, III.z: bullets weighed 542, 549 and 515 grains respectively (35.1, 35.6, 33.4 g), all at 2,470 fps (753 m/s). They all had a steel core with a tracer cavity drilled in the base, and all traced to 800 yards (730 m); the Mks II and III had a dark trace (i.e. tracer did not illuminate until 100 yards from muzzle). All were approved between 1940 and 1944. All had to achieve the same penetration figure as the SAP F Mk 1.z.
Incendiary B Mark I.z: bullet weighed 562 grains (36.4 g) and was similar in design to the .303 inch B Mk. VII (i.e., a simplified "De Wilde"). Within the jacket was a steel sleeve containing 28 grains of incendiary composition with a further 2 grains of QF composition held in the jacket tip. Introduced after 1939.
It appears that the AP rounds saw little use since production stopped prior to 1935. The SAP and SAP-T were increasingly used instead, particularly by the RN. This was presumably because they were almost as effective and considerably cheaper.
All of the bullets in British service were flat-based which limited their maximum ballistic range to 4,265 yards (3,900 m). They could achieve a maximum altitude of 9,500 feet (2,900 m). Of course, the effective ranges were very much less than this; they were probably in the region of 820 yards (750 m) against aircraft and 1,640 yards (1,500 m) against small surface targets.