plug 22 which with its own externally threaded shank 122 is screwed in a corresponding internally threaded portion of the upper end of the outer sliding tube 1 in the outer sliding 1 tube itself there is introduced a correct amount of a suitable liquid which preferably goes over the cap 19 crossing the clearance existing between the cap itself and the internal lateral surface of the inner sliding tube 8 for example up to the line of the level indicated with 23 in fig1 and such fluid completely fills the free chamber c of the outer sliding tube 1 and completely fills the barrel 13 in which it can flow through at least one lower hole 24 of suitable section the plug 22 is equipped with a gasket 25 for the seal connection on the inner sliding tube fig6 and is axially drilled for the passage of the upper tapered section 126 with lateral seal 27 of a screw 26 which is axially held in place by a snap ring 28 and which can be actuated through an external knob 29 on screw 26 is screwed a nut screw 30 externally equipped with teeth 130 which bind some corresponding longitudinal recesses 31 of the lateral surface of the axial cavity of the plug in which lodges the said screw u2014 nut screw group on the lower end of the nut screw 30 rests with the interpositioning of a truing cap 32 the upper end of the counter acting spring 21 by acting.