Just a FYI, the pertinent issue would be recoil velocity, not recoil energy.
Sorry, but you are incorrect. Velocity can only be achieved after an initial acceleration (velocity = distance/time, which is a steady state condition). In the case of a rifle shot, the initial energy is immediately reacted by the shoulder of the shooter, so no steady state condition ever exists until the rifle has been brought to a stop (acceleration followed by immediate deceleration).
Acceleration = force/mass (force of the chamber pressure pushing against the backside of the projectile divided by the mass of the system), therefore it is indeed the initial acceleration of the recoil (aka recoil energy), that imparts energy into the components of the rifle, such as an attached flashlight, and not the velocity.
The deceleration of the system back to the static condition provides the equal and opposite forces to satisfy the equations of equilibrium, but at a much more dampened rate compared to the initial acceleration.
In an attempt to keep this thread on topic, I would add that the deceleration of the rifle and its components into the shooter's shoulder would work to compress the spring of the flashlight, but as the shoulder reaction is dampened significantly to that of the initial acceleration, then no worries, especially with quality hardware like a Fenix light.
Best regards,
John C.