The Pershing was equipt with a large, double baffle muzzle brake. By "Expansion chamber" (?) believe you refer to the bore evacuator or fume extractor behind the single baffle muzzle brake on the later M-26A1 and M-46. The Pershing's gun was called a "single shot gun" due to the amount of dust kicked up by the hige mizzle brake, obscuring vision for a second shot. Due to the size of the 90mm shell casing with so much propellent, much of the burning residue was in the bore when the breechblock dropped, and flashed into the turret. A Pershing loader often had no eyebrows and a "sunburned " face. The Pershing's 90mm gun was modified by cutting the muzzle brake to single baffle to cut down on the back blast of dirt and the bore evacuator installed to cut down on the fumes. This was renamed M-26A1....same setup on the M-46. As the projectile leaves the gun, it creates a vacum in the bore evacuator which had angled holes into the bore. The vacuum would suck the fumes in until the pressure equalized and fumes exited out the muzzle. Hope this helps.