Spikes Tactical Bolt 5.56 Spikes HPT/MPI Bolt Assembly - Nickel Boron Finish. Whether you need a spare bolt or are looking to upgrade an existing rifle look no further than these Bolt Carrier Groups available with a Black Nitrite or Nickel Boron finish. This is the same bolt Spikes Tactical uses in their builds. Specifications for Spikes Tactical 5.56 HPT/MPI Bolt Assembly: Manufacturer: Spikes Tactical Caliber: 5.56x45mm NATO Condition: New Country of Origin: USA Features of Spikes Tactical 5.56 HPT/MPI Bolt Assembly: Mil Spec Bolt: Phosphate Finish 158 Carpenter Steel Shot Peened High Pressure Tested (HPT) Magnetic Particle Inspected (MPI) Mil-Spec Extractor 8620 Tool Steel Phosphate Finish Extractor Spring Black Extractor Insert Viton O-Ring Package Contents: Spikes Tactical 5.56 HPT/MPI Bolt Assembly Spikes Tactical ST5B503: Bolt 5.56 Spikes HPT/MPI Bolt Assembly - Nickel Boron Finish