base bleed

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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 (but does not produce thrust, unlike rocket-assisted projectiles). Being percentage-based, the range extension 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 kilometres (25 mi) 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.