Base bleed

Base bleed or base burn (BB) is a system used on some artillery shells to increase range, typically by about 20–35%. It expels gas into the low pressure area behind the shell to reduce base drag (it does not produce thrust; if it did it would be a rocket-assisted projectile). Since it 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, and it also was found that the reduced turbulence gave the projectiles a more consistent trajectory, resulting in tighter grouping and efficient shelling more than 40 km away.

Base bleed technology was developed in Sweden in the mid-1960s but took some time to spread and find its niche between cheaper classical ordnance and even more expensive rocket-assisted projectiles. It is now a fairly common option.