It would return to Earth with a lot less energy than it was fired with.

It would still have a lot of energy as it hit the ground. While it might have eventually lost altitude it would have increased potential energy at the top of its ballistic arc. The old V2 came in at supersonic speeds even though it only had enough energy to get it up. The return to earth had a lot of kinetic energy. I doubt that the projectile would have lost so much energy as to be safe when it landed.

I knew someone would say that.

I was even thinking of mentioning Mythbusters at the time of writing but that would probably trigger some correction of a mis-quote from the program or a typo or something so I didn't bother in the end.

I was going to say it would prob burn up on "re-entry" but from a stationary start it wouldn't be going fast enough through the atmosphere.

If the assumption is a perfect frictionless projectile, then it will land with the exact same energy it left with. The kinetic energy at the muzzle turns to gravitational potential energy at the peak of the trajectory, and back to kinetic at the point of impact.

If we take friction into account, then the entire exercise becomes horribly complicated but it's probably safe to assume that the impact will be close to the terminal velocity of the projectile at one atmosphere under one G.

Either way, I'd rather it didn't land on my head.