SAE 9310 MPI AR15 Bolt Assembly - .223 / 5.56 / 300BLK

Now: $40.00
(You save )


The Ground Zero Precision 5.56 AR15 bolt assembly is precision machined from SAE 9310 steel to exceed current U.S. military specifications and is shot peened for unsurpassed strength. Finished in durable magnesium phosphate for years of worry free service. These bolts come complete with a precision Swiss screw machined tool steel extractor, extractor spring, and every single one is Magnetic Particle Inspected (MPI), High Pressure Tested (HPT) and marked MPI or with an X indicating MPI.

Why SAE 9310 Steel? The Internet says Carpenter 158® is better.

Just like everything else, free advice from strangers on the Internet is worth exactly what you pay for it and in this case the "Internet experts" are flat wrong. The fact is that materials matter and the Carpenter 158® steel of the standard Mil-spec bolt has been proven to be sub-optimal for high-load applications such as that with 7.62x39 and 6.5 Grendel leading to a life expectancy of only about 6,000 rounds. In fact, actual use studies following the scientific method have repeatedly found that standard Carpenter 158® bolts will begin developing stress cracks on the locking lugs adjacent to the extractor cut after only 3,000 rounds and cracking at the cam pin hole and weak extractor spring tension only add to the problems of Mil-spec bolts under high-load applications. Standard Carpenter 158® bolts were fine enough for their time but it's a proven fact that the material used in these standard bolts is simply not the best choice by today's standards.

The SAE 9310 steel of our AR15 bolts is a superior steel alloy in virtually every way compared to the Carpenter 158®, whether in its resistance to structural failure or life expectancy, both of which far exceed current U.S. military specifications. There's a reason why Formula 1 racers choose SAE 9310 steel for their transmission gears over all other steel alloys available at their disposal. Like a Formula 1 transmission, the operational demands of a modern AR15 bolt assembly are ideally suited to this state-of-the-art material and actual full-auto endurance tests prove it.


  • Magnesium Phosphate (Mil-Spec)
  • Black Nitride (N3)
  • Nickel Boron (NiB)
  • Titanium Nitride (TiN)


  • Manufactured to Exceed U. S. Military Specification from SAE 9310 Steel
  • MPI Bolt (Magnetic Particle Inspected)
  • HPT Bolt (High Pressure Tested)
  • Precision Machined & Shot Peened Bolt
  • Tool Steel Extractor
  • Spring Steel Extractor Spring
  • Black Extractor Insert
  • Mil-Spec O-Rings
  • .223, 5.56, 300 Blackout and others compatible with the 5.56 bolt face



Additional Details


Related Products

Customers Also Viewed

Subscribe to our newsletter

Get the latest updates on new products and upcoming sales

No thanks