A clamp comprises opposed members having clamping surfaces defining a distance therebetween for receiving and clamping an object. The projectile is of the under-caliber type with respect to the size of the barrel for which, in order to correctly insert the projectile in the same, an adaptor is used, formed by a capsule fitting on the projectile, having an outer diameter which substantially corresponds to that of the barrel. The powertrain includes an engine and torque converter that is continuously connected to at least one of the planetary gear members and an output member that is continuously connected with another one of the planetary gear members.