What design feature of the Foster slug aids in its stability when fired from a shotgun?

The design feature of the Foster slug that aids in its stability when fired from a shotgun is the hollow rear that shifts the center of gravity. This specific design helps to enhance the slug's aerodynamic stability during flight. When the rear of the slug is hollow, it effectively lowers the center of gravity, which contributes to better balance and reduces the tendency to tumble as it travels through the air.

This design is particularly beneficial because it allows the slug to maintain a straight and stable trajectory, improving accuracy and effectiveness on target. The stability provided by the shifted center of gravity is critical, especially when the projectile is exiting the smooth bore of a shotgun barrel, as it ensures the slug behaves more like a traditional bullet.

Other design aspects, such as weight or surface texture, contribute to different functional characteristics of the slug, but it is the hollow rear feature that primarily focuses on flight stability after leaving the barrel.