Disclaimer: not an expert. Never played with colibri rounds. Just ran the numbers.
Given the ballistic similarity between a .22 pellet from a mid range (power wise) pellet rifle, and a .22 colibri, this may not be the best tool for armadillo dispatching. I've looked at the skeleton or a dillo and it looks like a .22 would kill it at full power, but barely. Anything less outside of a dispatching shot to the underside of the animal once it is trapped would probably be of little effect in the best case and crule in the worst case. I know we are talking about dillos here, but when I consider killing any animal in other than an emergency or self defense situation, I play this game in my head: I am going to be killed, I have no choice in the matter. What I do have a choice about is the method and qaulity of my death.
In this scenario, I'm going to be choose the fastest and least painful method I can. So I apply that to my decision of how to kill the animal.
But like I said, I've never done it, so a colibri may work just fine.