Abstract: A break chamber for a gas-blast circuit breaker comprising an insulating cylindrical housing (1) containing a fixed arcing contact (5), a fixed main contact (4), and a moving contact (6) passing through one end (3) of the housing. The housing is divided into a first portion (11) and a second portion (12) by a transverse partition. The first portion houses the fixed contacts. The second portion is itself subdivided into two adjacent zones of different diameters. The smaller diameter zone (12A) constitutes a cylinder for a piston (14) in the form of an annular disk fixed to the moving contact. The length of the smaller diameter zone is substantially equal to the length over which said fixed arcing contact (5) penetrates into said moving contact (6) when the circuit breaker is in its closed position.