Ironfoe was nerfed on Elysium prior to the merge with Nostalrius. The Nerf (Actually a blizzlike-fix) had been carried over to Nostalrius aswell. On Nostalrius it was exactly 10% before.   60/1020=0,0588235294117647 = 5.8% proc chance.       EDIT: Clarification. The test was carried out during a Full MC-clear with only Ironfoe used the entire run.