The tool comprises an assembly of a tip-guide and a shear unit, on which can be fastened a charger of fastening elements. The fastening elements are introduced in the bore of the tip-guide through a passing window provided in the shear unit. The tool comprises a pivoting flap for adjusting the effective length of the passing window to the length of the fastening elements to be driven. The flap is pivotally mounted about an axis substantially parallel to the tip-guide bore and is adapted to provide a guiding surface for the shortest fastening elements before they enter the partially plugged passing window. With the invention, the short elements can neither rock the tool nor jam it.