The M50 Self-propelled Howitzer, or "155mm Howitzer Motor Carriage, Tracked, M50", is the principle modern self-propelled artillery system of the U.S. Military. It is an armored mobility and enhancement platform for the M117 gun-howitzer (or rather, for the M118 variant of that weapon, which was developed specifically to be fitted into this vehicle). The M50 evolved out of a protracted development programme and incorporates all manner of advanced features which make it easily the most advanced system of its kind in the world (a distinction it is expected to retain for the forseeable future). Due to the large number of new and/or untested technologies in the design, it is a controversial weapons platform - but its revolutionary capabilities have done much to silence many of its detractors.
The program that would eventually mature into the M50 began in the early 1950s, alongside (or even before) the project for an air-drop capable 155mm howitzer which eventually led to the M117 Gun-Howitzer. The system which this program originally envisioned was markedly different from the M50, however: a light, fast, rapid-fire 105mm self-propelled artillery piece to provide highly mobile fire-support for armored / mechanized units, capable of rapidly emplacing, delivering fire, and then displacing to avoid counter-battery and keep pace with fast-moving armored advances.
Yet through the rest of the decade, conceptions of artillery requirements evolved considerably. 105mm Howitzers were no longer considered adequate as field artillery on the modern battlefield. Thus, a more broadly-effective 155mm system was needed. Early attempts to adapt the project to this requirement involved simply mounting a modification of the M114 - and later, M117 - guns on the existing chassis. However, the designers never really abandoned their dream of a rapid-fire artillery system, and development continued until this dream was brought to fruition.
The mature, fully-realized M50 Self-propelled Howitzer incorporates an advanced, complex autoloader mechanism based around a large three-round revolver mechanism and two six-round magazines. An electro-pneumatic machine feeds ammunition from the magazines into the revolver, while an automated ram chambers shells from this drum into the breech. The crew's loader, then, simply has to engage these two processes by turning levers. When firing at a single fixed set of coordinates, an M50 can pump out its 15 rounds (12 in the magazines plus 3 in the drum) in under a minute, i.e. at intervals of less than 4 seconds (the precise rate varies according to the individual vehicle's level of mechanical wear). The M50's integral crane can reload the magazines from a purpose-built ammunition carrier vehicle in less than three minutes. However, rounds can also be loaded directly into the revolver drum, by-passing the magazines, for situations where more flexibility or a more sustained rate of fire is required.