Tribological properties of projectiles, i.e. contact stability with the tube through which they pass, are of prime importance for the selection of soft or hard projectiles for on-line cleaning of tubular exchangers. To do so, this study proposes two methods of firstly measuring pressure in the back of projectiles under the force of flow and secondly the shear by mechanical force using a tensometer. The experimental results show that hard projectiles exert a much higher mechanical shear on the surface, nevertheless much lower hydrodynamic shear under the propulsion of flow. This is mainly due to reduced and unstable contact area between the projectile and tube surface. Accordingly a new term contact stability or Z factor is proposed which has a strong relation to the stiffness of projectiles. The Z factor is low for hard projectiles and vice versa. A set of fouling runs, under similar operating conditions, is also conducted for hard and soft projectiles as well as when no projectile is shot. While a hard projectile can exert a shear of 12-fold larger than a soft projectile, its cleanability is not appreciably better. The reason was attributed to poor contact stability with the tube surface.