the same chassis could be. They generally fell into three overlapping categories: improvised modifications of old or captured tanks to render them viable again (such as converting the machine-gun-only Panzer I into the Panzerjäger I), often with haphazard, poorly protected, limited-traverse weapon mounts; the American offensive and mobile reserve model, which favoured lightly-armed open-top vehicles with a rotating turret and a powerful anti-tank-capable gun while relegating true tanks to infantry support role (exemplified by the M10 tank destroyer); and the casemate gun mount model, which often allowed the resultant vehicle to be hard to hit and have a well-sloped and heavily armoured glacis plate (for instance, the SU-100). The relative superiority in armament of tank destroyers was only relative, however: for instance, the SU-85 was a casemate-type TD on the T-34 chassis that was rendered obsolete once the basic T-34 switched from the 76 mm gun to the same 85 mm cannon, producing the T-34-85. After World War II By the end of the war the variety in tank designs had narrowed and the concept of the main battle tank emerged. The race to increase caliber slowed, with just slight increases between tank generations. In the West, guns of around 90 mm gave way to the ubiquitous 105 mm Royal Ordnance L7, introduced in 1958. This lasted a long while, with a shift to 120 mm in the late 1970s and early 1980s (the UK changed in the late 1960s with their Chieftain tank). In the East, the 85 mm quickly yielded to the 100 mm and 115 mm U-5TS gun, with the 125 mm caliber now standard. Most of the improvements were instead made in ammunition and fire-control systems. With kinetic energy penetrator rounds, solid shot and armour-piercing shell gave way to armour-piercing discarding sabot (APDS) (a product of 1944), and fin-stabilized (APFSDS) rounds with tungsten or depleted uranium penetrators. Parallel developments brought rounds based on chemical energy; high-explosive squash head (HESH), and shaped-charge high-explosive anti-tank (HEAT), with penetrating power independent of muzzle velocity or range. Stadiametric range-finders were successively replaced by coincidence and laser rangefinders. Accuracy of modern tank guns is improved over earlier weapons by computerized fire-control systems, wind sensors, thermal sleeves, and muzzle referencing systems which compensate for barrel warping, wear and temperature. Fighting capability at night, in poor weather, and smoke was improved by infrared, light-intensification, and thermal imaging equipment. Technology of the guns themselves