Real 11-87 Premier or newer Sporstman with walnut stock.....How old is the gun, did you break the original and two others? Did you order the replacement part from Remington or an after market supplier. I have noticed some parts from Numrich and Brownwell's not exactly up to factory specs.
Has the break happened in the same place, that to me would indicate something is happening with the bolt, maybe binding, any abnormal ware on any other internal parts.
Is the gas piston assembly installed properly, could you have one of the three parts reversed by accident, this may cause the bolt to recoil to fast, putting stress on the action bar assembly if it strikes the receiver to fast.