Final answer:
The assembly that moves inside the barrel-shaped body of a cylinder is the piston, which is actuated by a spring in some mechanisms, converting elastic potential energy into kinetic energy.
Step-by-step explanation:
The assembly that moves inside the barrel-shaped body of a cylinder is the piston.
In the context of a gun, a spring may be used to provide the force needed to move the piston, but it does not move inside the cylinder. Instead, the spring is usually part of the mechanism that allows the piston to move back and forth within the cylinder. When the spring is compressed and then released, it converts elastic potential energy into kinetic energy, causing the piston to move and often to compress gas, as is the case in internal combustion engines or to push a projectile, as in a firearm.