If the thought of the possibility to detach yourself from the ground by firing bullets down at it has brought up concerns about your sense of reality, you will now be relieved.
Randal Munroe, an XKCD researcher has proved the idea has a good scientific explanation, Gizmodo
“The principle here is pretty simple. If you fire a bullet forward, the recoil pushes you back. So if you fire downward, the recoil should push you up,” declared Prof. Munroe.
Obviously, the entire process is by far more complex and needs more advanced techniques.
“[O]ur optimal craft comprises a large number of AK-47s (a minimum of 25 but ideally at least 300) carrying 250 rounds of ammunition each. The largest versions of this craft could accelerate upward to vertical speeds approaching 100 meters per second, climbing over half a kilometer into the air,” explained Prof. Munroe sophisticatedly.
But the main point is: Yes, it can be done!