They were at one point metal, then supposedly the military requested the polymer, fluted rod be used instead because the metal rods can warp. This was to enhance reliability in sandy adverse conditions. The fluted rods still function when covered in dirt, sand, etc. because the grooves give the dirt a place to go without interfering with the recoil spring's action. Also, with the polymer's "self-lubricating" properties you don't have to oil the rod since oil attracts dirt. This was told to me by a Beretta rep. over the phone, anyway.