Yes, the only sensible way to quote the energy of any kind of gun, it to quote the energy imparted to the projectile. Some of that energy will be in the form of heat, but most will be kinetic energy.
The failure of the barrel has nothing to do with the level of energy in the projectile per se - though they are hardly independent. Failure of the barrel happens when the power (energy over time) is sufficient to spall the crystals. Strain hardened crystalline structures are much more resilient against this type of load, but fatigued structures are not.
As I say, I can discuss the theory all day long, but I have very limited practical experience. I am hoping to learn from those on this form with experience, and I hope to contribute in some way too