[beebuster2000]

why do all tank guns have the bulge half way up the barrel? what does it do?

[FreedomPoster]

Fume extractor. Holes in the barrel allow propellant fumes to exit and diffuse from there. Beats the heck out of propellant fumes in the crew compartment upon opening the breech to reload.

[MasterGunner01]

The ‘hump’ in the middle of the barrel is a a bore evacuator that sucks out the gun gas (burned propellant gas) and ejects it from the muzzle. Gun gas is both toxic and flammable. Large naval guns use compressed air to clear out the gun gas when the breech block opens after firing to purge the bore.