Base bleed is a system used on some artillery shells to increase their range, typically by about 20–35%. Since base bleed extends the range by a percentage, it is more useful on longer range artillery where an increase of approximately 5–15 kilometres (3.1–9.3 mi) can be achieved. Until the late 1980s the small gains in range were not considered worthwhile for field artillery. Base bleed shells are becoming more common in units equipped with modern artillery which have far greater range than the old ones, but are usually only used at close to, and beyond, normal maximum ranges due to the higher cost per shell.
Most (50–60%) of the drag on an artillery shell comes from the nose of the shell, as it pushes the air out of its way at supersonic speeds. Shaping the shell properly can reduce this drag but it is difficult to remove.
However, another powerful source of drag is the low-pressure area left behind the shell due to its blunt base. Base bleed can reduce this drag without extending the base of the shell. Instead, a small ring of metal extends just past the base, and the area in the rear of the shell is filled with a small gas generator. The gas generator provides little to no thrust, but fills the vacuum in the area behind the shell with an inflow of gas, dramatically reducing the drag. The lessened turbulence also turned out to produce tighter grouping due to the projectiles having a more consistent trajectory. The only disadvantage, apart from a higher price per shell, was a small loss in explosive payload in older shells due to some of the space inside the shell being taken up by this mechanism, but modern gas generators are both smaller and incorporated in the shell casing.
The principles were developed in Sweden in the mid 1960s, by the Försvarets forskningsanstalt (abbreviated FOA), and the Artillery bureau at the Kungliga Materielförvaltningen (later the Försvarets Materielverk (FMV)) while working on a rocket-assisted projectile called "reatil". Their goal was to increase the range of coastal artillery. By 1966, it had been concluded that a small, slow-burning charge at the base of the projectile would alleviate the low pressure behind the shell, hence increasing the range by lessening the difference between the pressure due to aerodynamic drag on the nose of the shell and the low pressure behind the base. The first full-scale tests took place in 1969 with modified 10.5 cm steel shells with excellent results, and the Swedish patent was granted FOA in 1971 although both application and patent were classified. Since the development was done, the patent was transferred to FMV for procurement to the armed forces of Sweden.
The concept was quickly implemented into the 7.5 cm sjömålsgranat m/66 (7.5 cm anti-shipping shell m/66) used in the 7.5 cm tornpjäs m/57 fixed coastal artillery gun, and then rapidly into all anti-shipping shells in the Swedish military.
Since FMV was to contract a company in the US to manufacture the gas generator for the 12 cm sjömålsgranat m/70 (12 cm anti-shipping shell m/70), used in the 12 cm TAP m/70 fixed coastal artillery gun, the classification secret was removed from the patent. Shortly thereafter the international rights were sold, eventually ending up with the Space Research Corporation (SRC), then owned by aeronautical engineer Gerald Bull.
At the end of the 20th century, the technology was generally available world-wide.