Base bleed is a system used on some artillery shells to increase their range, typically by about 30%.
Most 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 greatly. However, another powerful source of drag is the vacuum left behind the shell due to its blunt base. This drag is difficult to remove, because the shell needs to be "nose heavy" in order to have proper ballistics, and it cannot easily be shaped into a more aerodynamic form.
Base bleed is one way to 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 net thrust, but simply fills the area behind the shell with pressure, dramatically reducing the drag due to the vacuum. The only downsides are a small loss of accuracy due to the somewhat more turbulent airflow, and a small loss in explosive payload due to some of the space inside the shell being taken up by this thrust mechanism.
Since base bleed extends the range by a percentage, it is only really useful on longer range artillery. Until recently the loss in explosive was not considered worthwhile for the small gains in range. However, the introduction of much longer range systems based on the GC-45 howitzer has changed the equation somewhat, as 30% extra range on these systems represents 5 to 10 km. Base bleed shells are starting to become more common in units equipped with modern artillery of this type.
The principles were developed in Sweden in the late 1960s, by the Swedish Defence Research Agency and the artillery bureau at the Defence Materiel Administration. 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 reduce the vacuum behind the shell, hence increasing the range due to lower drag. The first full scale tests took place in 1969 with modified 10.5 cm steel shells with excellent results, and the Swedish patent filed in 1971.
The concept was quickly implemented into the 7,5 cm sjömålsgranat m/66 (anti-shipping shell m/66) used in the coastal fortification system 7,5/m57, and then rapidly into all anti-ship shells in the Swedish military.
Since the gas generator for the 12 cm sjömålsgranat m/70 was to be manufactured by a company in the USA, the classification "secret" was removed from the patent. Shortly thereafter, the international rights were sold out ending up with the Space Research Corporation (SRC), owned by Gerald Bull.