A Glock will certainly fire with the slide immobilized. If the slide cannot move with respect to the frame the current round will not be ejected and the next round will not be chambered, but it will certainly fire the chambered round. It is not clear from "grab the whole barrel" what you were doing. Glock is like many (most?) semiautos in that the trigger cannot be pulled when the slide is not all the way forward, is that what you meant? That you could yank the slide out of battery and hold it there, thus preventing the trigger from being pulled?

If you intend to replicate your experiments in the future, you can explicitly seat a snap cap in the chamber and leave the magazine out (no mag safety on the Glock). That way you are still violating rules 2-4, but possibly not rule 1.