The K98 Grenade Launcher from the Secret Weapons expansion has always displayed unresponsive behavior. By that, I mean the fact that it never bounces and moves about like it's covered in grease.
It confounds me too but after some experiments, I have found a clue to the possible source.
In the projectile section of the weapon, setting ObjectTemplate.geometry to something other than its native projectile model, such as the geometry for the normal grenades, suddenly made the projectile actually respond. It bounces and actually slows down when touching the ground.
With some more poking around, I conclude that the mesh file for the projectile lacks crucial data to actually interact with its environment.