A drum comprising: an outer cylindrical shell defining a hollow housing having opposing open lower and upper ends (13, 14); a lower and upper skin (15, 16) each covering a respective one of the lower and upper ends of said outer cylindrical shell; and a diameter adjustment mechanism having at least one inner cylindrical shell (21, 22), wherein each inner cylindrical shell (21, 22) has an upper rim, said diameter adjustment mechanism being configured to be positioned inside the hollow housing and fixedly coupled to the inner surface of the outer cylindrical shell, wherein the diameter adjustment mechanism is configured for axial displacement of the at least one inner cylindrical shell (21, 22) and wherein the upper rim of said at least one inner cylindrical shell (21, 22) can be brought into or out of contact with the upper skin to thereby change an effective diameter of the upper skin, characterized in that the diameter adjustment mechanism further comprises: a rotatable rim supporting at least one ramp for each inner cylindrical shell, the ramp is configured to rotate relative to the outer cylindrical shell of the drum and being dimensioned such that when a lower rim of the respective inner cylindrical shell (21, 22) is supported at an upper end of the ramp, the upper rim abuts the upper skin and when the lower rim of the respective inner cylindrical shell (21, 22) is supported at a lower end of the ramp, the upper rim is clear of the upper skin a linear bevel gear mounted toward a periphery of the rotatable rim, and a pinion gear rotatably mounted toward the lower end of the cylindrical shell for engaging the linear bevel gear; wherein rotation of the pinion gear in a first direction rotates the rotatable rim in a first direction and causes the ramp to push the inner cylindrical shell (21, 22) upward, while rotation of the pinion gear in a second opposite direction rotates the rotatable rim in a second opposite direction, thereby lowering the ramp and allowing the inner cylindrical shell (21, 22) to fall clear of the upper skin. an outer cylindrical shell defining a hollow housing having opposing open lower and upper ends (13, 14); a lower and upper skin (15, 16) each covering a respective one of the lower and upper ends of said outer cylindrical shell; and a diameter adjustment mechanism having at least one inner cylindrical shell (21, 22), wherein each inner cylindrical shell (21, 22) has an upper rim, said diameter adjustment mechanism being configured to be positioned inside the hollow housing and fixedly coupled to the inner surface of the outer cylindrical shell, wherein the diameter adjustment mechanism is configured for axial displacement of the at least one inner cylindrical shell (21, 22) and wherein the upper rim of said at least one inner cylindrical shell (21, 22) can be brought into or out of contact with the upper skin to thereby change an effective diameter of the upper skin, characterized in that the diameter adjustment mechanism further comprises: a rotatable rim supporting at least one ramp for each inner cylindrical shell, the ramp is configured to rotate relative to the outer cylindrical shell of the drum and being dimensioned such that when a lower rim of the respective inner cylindrical shell (21, 22) is supported at an upper end of the ramp, the upper rim abuts the upper skin and when the lower rim of the respective inner cylindrical shell (21, 22) is supported at a lower end of the ramp, the upper rim is clear of the upper skin a linear bevel gear mounted toward a periphery of the rotatable rim, and a pinion gear rotatably mounted toward the lower end of the cylindrical shell for engaging the linear bevel gear; wherein rotation of the pinion gear in a first direction rotates the rotatable rim in a first direction and causes the ramp to push the inner cylindrical shell (21, 22) upward, while rotation of the pinion gear in a second opposite direction rotates the rotatable rim in a second opposite direction, thereby lowering the ramp and allowing the inner cylindrical shell (21, 22) to fall clear of the upper skin. a rotatable rim supporting at least one ramp for each inner cylindrical shell, the ramp is configured to rotate relative to the outer cylindrical shell of the drum and being dimensioned such that when a lower rim of the respective inner cylindrical shell (21, 22) is supported at an upper end of the ramp, the upper rim abuts the upper skin and when the lower rim of the respective inner cylindrical shell (21, 22) is supported at a lower end of the ramp, the upper rim is clear of the upper skin a linear bevel gear mounted toward a periphery of the rotatable rim, and a pinion gear rotatably mounted toward the lower end of the cylindrical shell for engaging the linear bevel gear; wherein rotation of the pinion gear in a first direction rotates the rotatable rim in a first direction and causes the ramp to push the inner cylindrical shell (21, 22) upward, while rotation of the pinion gear in a second opposite direction rotates the rotatable rim in a second opposite direction, thereby lowering the ramp and allowing the inner cylindrical shell (21, 22) to fall clear of the upper skin.