New hydrodynamic swing arm that rests on the shaft for the entire length of the barrel during the pushing phase of the tires. The arch is wider than traditional arched cross arms which keeps the bands parallel and is easier to load. Inserted in the arches of the forks are two cone-shaped couplings in polyurethane that have a double function. Accidental opening of the cross arm after the shot is being avoided. It also avoids scratching the steel part on the barrel thus also avoiding any noise. It is available in two versions: for notched trees and shark fin trees.