A pneumatically operated drill hammer comprising a piston, a distributor, and a housing, wherein the piston and the distributor are axially arranged relative each other in the housing, the piston being arranged to be moved axially between a first position and a second position, wherein the drill hammer further comprises a pneumatic drive system comprising a drive chamber, a return chamber and a plurality of channels for distribution of drive gas in said drive system, the return chamber being arranged at a lower side of the piston and the drive chamber being arranged at an upper side of the piston, between the piston and the distributor, wherein the drive chamber is defined by a variable space enclosed by at least the housing, the piston and the distributor, wherein the drive chamber comprises at least one inlet port and at least one exhaust port arranged at a circumference of the drive chamber, wherein pressurization of the pneumatic drive system alternates pressurization of the drive chamber and the return chamber, wherein the piston moves back and forth between the first and second position by means of said alternating pressurization of the drive chamber and return chamber, and wherein the piston comprises a first inner surface, and the distributor comprises a second inner surface, said first and second inner surfaces (27, 29) being arranged to align adjacent each other when the piston moves towards the first position, wherein said alignment divides the drive chamber into a sub-drive chamber and at least one boost chamber, said at least one boost chamber being separated from the inlet and the exhaust port (21, 23) and wherein at least one of the inner surfaces (27, 29) comprises a bevelled edge, arranged at a side of said inner surface (27, 29) that is aligned with another inner surface (27, 29) to form the boost chamber when the piston moves towards the first position. wherein the drill hammer further comprises a pneumatic drive system comprising a drive chamber, a return chamber and a plurality of channels for distribution of drive gas in said drive system, the return chamber being arranged at a lower side of the piston and the drive chamber being arranged at an upper side of the piston, between the piston and the distributor, wherein the drive chamber is defined by a variable space enclosed by at least the housing, the piston and the distributor, wherein the drive chamber comprises at least one inlet port and at least one exhaust port arranged at a circumference of the drive chamber, wherein pressurization of the pneumatic drive system alternates pressurization of the drive chamber and the return chamber, wherein the piston moves back and forth between the first and second position by means of said alternating pressurization of the drive chamber and return chamber, and wherein the piston comprises a first inner surface, and the distributor comprises a second inner surface, said first and second inner surfaces (27, 29) being arranged to align adjacent each other when the piston moves towards the first position, wherein said alignment divides the drive chamber into a sub-drive chamber and at least one boost chamber, said at least one boost chamber being separated from the inlet and the exhaust port (21, 23) and wherein at least one of the inner surfaces (27, 29) comprises a bevelled edge, arranged at a side of said inner surface (27, 29) that is aligned with another inner surface (27, 29) to form the boost chamber when the piston moves towards the first position.