Method for forming a collar in a muffler shell for a muffler for an internal combustion engine, the method comprising: - providing a muffler shell made of a metal sheet and forming a muffler housing, the muffler shell having an exhaust gas opening, - providing a collar forming head having a rotational axis and at least two movable expanders, - introducing the collar forming head into the muffler housing, wherein the expanders are in a retracted position, by moving the retracted collar forming head along the rotational axis through the exhaust gas opening in the muffler shell, wherein the retracted collar forming head has a largest diameter smaller than the largest diameter of the exhaust gas opening in the muffler shell, - moving the expanders of the collar forming head introduced into the muffler housing radially away from the rotational axis of the collar forming head from the retracted position to an expanded position, in which the largest diameter of the expanded collar forming head radially to the rotational axis is larger than the largest diameter of the exhaust gas opening, - rotating the collar forming head around the rotational axis of the collar forming head, - bringing the rotating expanded collar forming head in contact with the metal sheet adjacent to and surrounding the exhaust gas opening in the muffler shell by moving the expanded collar forming head along the rotational axis in a direction outwardly from the interior of the muffler housing, - forming an outwardly projecting collar around the exhaust gas opening by flaring the edge of the metal sheet surrounding the exhaust gas opening outwardly and increasing the size of the exhaust gas opening by moving the expanded collar forming head along the rotational axis in a direction outwardly from the interior of the muffler housing to an exterior of the muffler housing. - providing a muffler shell made of a metal sheet and forming a muffler housing, the muffler shell having an exhaust gas opening, - providing a collar forming head having a rotational axis and at least two movable expanders, - introducing the collar forming head into the muffler housing, wherein the expanders are in a retracted position, by moving the retracted collar forming head along the rotational axis through the exhaust gas opening in the muffler shell, wherein the retracted collar forming head has a largest diameter smaller than the largest diameter of the exhaust gas opening in the muffler shell, - moving the expanders of the collar forming head introduced into the muffler housing radially away from the rotational axis of the collar forming head from the retracted position to an expanded position, in which the largest diameter of the expanded collar forming head radially to the rotational axis is larger than the largest diameter of the exhaust gas opening, - rotating the collar forming head around the rotational axis of the collar forming head, - bringing the rotating expanded collar forming head in contact with the metal sheet adjacent to and surrounding the exhaust gas opening in the muffler shell by moving the expanded collar forming head along the rotational axis in a direction outwardly from the interior of the muffler housing, - forming an outwardly projecting collar around the exhaust gas opening by flaring the edge of the metal sheet surrounding the exhaust gas opening outwardly and increasing the size of the exhaust gas opening by moving the expanded collar forming head along the rotational axis in a direction outwardly from the interior of the muffler housing to an exterior of the muffler housing.