Key to the effectiveness of a HEAT round is the width of the warhead. As the penetration continues through the armor, the width of the hole decreases leading to a characteristic "fist to finger" penetration, where the size of the eventual "finger" is based on the size of the original "fist". In general HEAT rounds can expect to penetrate armor of 150% to 250% of their width. HEAT rounds are made less effective if they are spinning, requiring modifications when fired from rifled guns.
HEAT systems were first introduced by the US as the bazooka, and soon followed by the British PIAT. The Germans were quick to realize the potential of the HEAT design, introducing a larger copy of the bazooka as the Panzerschreck, and a feared single-use system known as Panzerfaust. These systems were all short-range, meaning that combined arms could reduce their effectiveness on the battlefield.
The need for a large bore made HEAT rounds ineffective in existing anti-tank guns of the era. The Germans were able to capitalize on this, however, introducing a round that was placed over the end on the outside of their otherwise outdated (and basically useless) 37mm anti-tank guns to produce a medium-range low-velocity weapon. A more convincing system was created by making a much larger tripod-mounted version of the Panzerschreck, producing what is today known as a recoilless rifle. Introduced in the very late stages of the war, the recoilless rifle had the range to stay easily hidden on the battlefield, was light enough to be portable by a small team, but had the performance needed to defeat any tank.
Adaptations to existing tanks were somewhat more difficult, although all major forces had done so by the end of the war. Since velocity has little effect on the armor-piercing capability of the round, which is defined by explosive power, HEAT rounds were particularly useful in long-range combat. The Germans were again the ones to produce the most capable gun-fired HEAT rounds, using a driving band on bearings to allow it to fly unspun from their existing rifled tank guns. HEAT was particularly useful to them because it allowed the low-velocity large-bore guns used on their numerous assault guns to become useful anti-tank weapons as well.
HEAT rounds caused a revolution in anti-tank warfare when they were first introduced in the later stages of World War II. A single infantryman could effectively kill any existing tank with a handheld weapon, thereby dramatically altering the nature of mobile operations. After the war HEAT became almost universal as the primary anti-tank weapon. When combined with the wire-guided missile, infantry weapons were able to operate in the long-range role as well. Anti-tank missiles altered the nature of tank warfare throughout the 1960s and into the 80s, and remain an effective system today. However, increased size and changes to the armor of main battle tanks have reduced the usefulness of HEAT to a degree, by making the needed warhead size large enough to be no longer man portable. Today HEAT rounds are primarily used in shoulder-launched and in jeep- and helicopter-based missile systems. (Main battle tanks use the more effective APFSDS rounds)
With the effectiveness of gun-fired HEAT rounds being lessened by the increasingly sophisticated armor, a new class of HEAT rounds known as high explosive dual-purpose, or HEDP, has become popular. These are essentially HEAT rounds effective against older tanks and other armored vehicles, with enough explosive to have a reasonable burst effect that can be used in place of conventional explosive rounds in anti-infantry roles. This reduces the total number of rounds that need to be carried for different roles.
The reason for the ineffectiveness of HEAT-munitions against modern main battle tanks can be contributed in part to the use of reactive armor, and to the development of multi-layer composite armor. The jet which is created by the explosion of the HEAT-round must have a certain distance from the target and must not be deflected. When the HEAT round hits the reactive armor and the explosion is ignited, the reactive armor will create an explosion of its own causing the jet to deform and so penetration power is greatly reduced. Alternatively, composite armor featuring ceramics erode the liner jet more quickly than rolled homogenous armor steel, the then preferred material in the construction of armored fighting vehicles.
Most HEAT-missiles today have 2 separate warheads to be more effective against reactive armor: The first, smaller warhead neutralizes the reactive layer while the second, larger warhead penetrates the normal armor below.