Air rifle power (specifically muzzle energy) is typically measured in either Joules or Ft-Lb. I never understood how these could be equivalent.

The crux of the confusion comes from the fact that a pound can describe both a weight and a force. Really it should be called 'foot pound-force' or 'ft-lbf'. From Wikipedia:

Pound-force should not be confused with pound-mass (lb), often simply called "pound", which is a unit of mass; nor should these be confused with foot-pound (ft⋅lbf), a unit of energy, or pound-foot (lbf⋅ft), a unit of torque.

What a mess.

Now it makes sense: ft⋅lbf = force⋅distance, and Joule = newton⋅meter = force⋅distance, therefore ft-lb is equivelent to Joules.

The moral of the story: Always use metric and SI units.