In a pyrotechnic linear piston and cylinder drive in a belt pretensioner, the cylinder (10) has a frustoconical free end (12) and the piston (14) is attached to the end of a cable (16). The piston (14) is tapered frustoconically at its end adjacent to the free end (12) of the cylinder (10). An annular piston member (22) is slipped over and frictionally engaged with the tapered end portion (20) of the piston (14). When the annular piston member (22) reaches the frustoconical free end (12) of the cylinder (10), the piston (14) is forced further into the annular piston member, deforming both parts and dissipating energy.