What does the term "piggyback" refer to in the context of bullet trajectories?

In the context of bullet trajectories, the term "piggyback" specifically refers to two bullets that exit a firearm at nearly the same time and in close proximity to each other, traveling in tandem. This phenomenon can occur when two projectiles are fired simultaneously from a weapon, allowing them to share the same trajectory for a short distance before any deviation occurs.

This concept is crucial in forensic ballistics, particularly when analyzing crime scenes, as it can provide insights into the firing mechanism used and the potential for multiple shots fired in quick succession. Understanding this helps forensic experts to reconstruct events accurately, identify if a firearm was modified, or even inform on the type of ammunition used.

The other options do not capture the essence of the term "piggyback" as it is used in firearms and ballistics. For example, firing two bullets from a single chamber or having two bullets that travel side by side may occur under specific circumstances, but they do not denote the simultaneous aspect implied by "piggyback."