In addition to mikeash's correction, a round does not contain everything that you need. Without a chamber, a bullet is not going anywhere at a particularly dangerous speed. The casing is really not meant to contain the pressure of the burning propellant by itself and without a barrel then the expanding gases from the burning propellant would primarily dump their energy into the environment, not the bullet. In fact, without a chamber and barrel, the casing is actually more dangerous than the bullet, though still would not be deadly. This makes sense when you consider that the casing weights far less than the bullet.

The "tube" is essential, not merely convenient.

You're thinking of the cartridge. The bullet is just the projectile, and does not include the propellant.