The large bulge on the rear of the barrel towards the mantlet is the bore evacuator/fume extractor. It sucks the excess smoke and gunpowder fumes out of the barrel and releases it outside the tank so the crew doesn’t have to inhale it.
Surrounding the entire barrel? Thermal sleeve. Its meant to keep barrel warp down in extreme cold weather. Because the barrel is made of metal, it reacts to changes in heat and cold, which can effect the boresight and accuracy of the gun.
The device towards the muzzle? That is the muzzle reference system. (MRS). It measures barrel bend in order to tank into account how it affects accuracy when firing. The information is processed by the tank’s FCS computer. (Fire control system)
And then there is the barrel itself, which sends 120mm sized projectiles of varying types downrange at the enemy.