ERFB:
The extended range, full bore concept reverses the normal rifling idea by using a projectile with small fins that ride in the grooves, as opposed to using a slightly oversized projectile which is forced into the grooves.
Notice the small spine on either side of the shell? Those are principle to the ERFB concept.
Base Bleed:
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 to no thrust, but simply fills the vacuum behind the shell with pressure, dramatically reducing the drag. 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.
An attachable Base Bleed unit. Also, notice the lack of spine on this shell.
Sometimes you'll find a shell that combines the two concepts, the ERFB-BB:
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