If it was long enough, I would buy a 3/16" diameter steel or brass rod. Mild steel would be strong enough if your using it correctly and the finish ground rods available at some hardware stores would work even better.
As you know, when loading a muzzleoader with a ramrod, the rod is grasped about 8" to 10" above the muzzle and shoved down the barrel. It is then grabbed again 8" to 10" above the muzzle and the task is repeated until the ball is seated on the powder.
Used in this fashion, there should be no chance of bending it.