MPI AR Bolt and Barrel
- Thread starter btmj
- Start date
Crow Hunter
New member
Depends.
What are you going to use it for?
If the worst thing that can happen to you if you break a bolt lug or have a bolt fail at a cam pin hole is that you have to go home early from a day at the range. There really isn't much advantage to it.
A rifle that does not have a MPI tested bolt has no "proof" that there are no manufacturing defects that could cause an early failure.
Are you willing to take that chance with your life?
I was not.
I have worked as a Mechanical/Manufacturing Engineer since 1997. 7 years of that was making brake rotors for Ford/Dodge/GM & Nissan pickups. Manufacturing defects DO happen and if you aren't checking for them, you won't know there is a problem.
What are you going to use it for?
If the worst thing that can happen to you if you break a bolt lug or have a bolt fail at a cam pin hole is that you have to go home early from a day at the range. There really isn't much advantage to it.
A rifle that does not have a MPI tested bolt has no "proof" that there are no manufacturing defects that could cause an early failure.
Are you willing to take that chance with your life?
I was not.
I have worked as a Mechanical/Manufacturing Engineer since 1997. 7 years of that was making brake rotors for Ford/Dodge/GM & Nissan pickups. Manufacturing defects DO happen and if you aren't checking for them, you won't know there is a problem.
madcratebuilder
New member
There is more to the mil-spec that just the destructive testing. I place more importance on the material and manufacturing process. The bolt carrier needs to be a specific grade of steel, receive a specific shoot peening procedure and heat treatment. The bolt needs to be C158, the HPT and MPI is a added plus.
The cost difference between a mil-spec and commercial grade BCG is about $25-30. If buying new it doesn't make sense to buy commercial grade to save 20%, but I'm not going to replace the two commercial grade BCG's I have until they fail.
Aero Precision is making BCG's now. I believe they are mil-spec material, 8620/C158 but not HPT or MPI tested, just batch tested, under $100. I may try one of these for a target build I have going.
The cost difference between a mil-spec and commercial grade BCG is about $25-30. If buying new it doesn't make sense to buy commercial grade to save 20%, but I'm not going to replace the two commercial grade BCG's I have until they fail.
Aero Precision is making BCG's now. I believe they are mil-spec material, 8620/C158 but not HPT or MPI tested, just batch tested, under $100. I may try one of these for a target build I have going.
Another perspective is, why does the .Gov need them individually inspected? Because, they aren't shipping them back to the dealer for Customer Service to fix. Getting the bolts MPI'd up front reduces a lot of them failiing early - which is already a small number anyway.
Note, I did not say they would never fail. The AR bolt is known to get cracks in the lugs after 5,000 rounds - 5,000 full power military rounds - which the service then replaces if and when they finally fail. There is NO program to do it before then, it's all up to the operator bothering to look during their daily PMCS. Once in a blue moon the armorer will pull them and check, they aren't doing much of that unless needed, they've got a plate full with extra duties.
A military grade weapon has to endure burst or full auto fire, with full power ammo, and that means it needs to be right from the first magazine. However, in civilian sales, the maker/assemblers know it's pricey, that their customers may likely take up to a decade to shoot that much ammo, it won't likely be full power at all, just cheap white box or import. They play the odds.
Be advised so does Colt. They have Customer Service, too, as do ALL the close-to-milspec-vendors out there. Even if they sell bolts marked MPI.
If anything, about the only bolts on the market with MPI inspection of every one sold are the AR15's. I seriously doubt it's done with many civilian guns, I've never seen the markings anywhere else. If it could sell more guns, the marketing department would do it if it made a buck.
Mine was a 9510 alloy superbolt, and I don't see the marking. Likely won't shoot 5,000 rounds thru it in my entire life. I preferred nitriding as a superior feature and got that. It goes to what you intend to do with the rifle.
Note, I did not say they would never fail. The AR bolt is known to get cracks in the lugs after 5,000 rounds - 5,000 full power military rounds - which the service then replaces if and when they finally fail. There is NO program to do it before then, it's all up to the operator bothering to look during their daily PMCS. Once in a blue moon the armorer will pull them and check, they aren't doing much of that unless needed, they've got a plate full with extra duties.
A military grade weapon has to endure burst or full auto fire, with full power ammo, and that means it needs to be right from the first magazine. However, in civilian sales, the maker/assemblers know it's pricey, that their customers may likely take up to a decade to shoot that much ammo, it won't likely be full power at all, just cheap white box or import. They play the odds.
Be advised so does Colt. They have Customer Service, too, as do ALL the close-to-milspec-vendors out there. Even if they sell bolts marked MPI.
If anything, about the only bolts on the market with MPI inspection of every one sold are the AR15's. I seriously doubt it's done with many civilian guns, I've never seen the markings anywhere else. If it could sell more guns, the marketing department would do it if it made a buck.
Mine was a 9510 alloy superbolt, and I don't see the marking. Likely won't shoot 5,000 rounds thru it in my entire life. I preferred nitriding as a superior feature and got that. It goes to what you intend to do with the rifle.
MPI is a technique used to detect cracks, inclusions, or voids in ferrous metallic structures. Normally it is used to inspect a structure or part after it has been in service for a long period of time, i.e. to detect fatigue cracks. When it is used on a newly-manufactured part (as in an AR-15 bolt), the purpose is to detect an initial flaw.
An AR-15 bolt is made from high strength hardened steel, current Mil-Spec calls for Carpenter 158 (originally a steel for molding plastic). Due to the nature of this class of steel and the high operating stresses these parts are designed for, a flaw at a high stress (or high Kt) area will quickly grow into a crack, and that crack will propagate through the part until it reaches a low stress area, or until the part fractures. The complex geometry of an AR-15 bolt makes fracture more likely. Bottom line, if a bolt has an initial flaw, failure of the bolt will probably occur sometime between the first few hundred rounds.
A bolt that is purchased by the DOD may very well be used in combat from the very start of its life. The bolt could be used in a new weapon issued to a soldier or marine in a theatre of operations, or it could be a replacement bolt used to repair a weapon. Either way, the DOD can not count on several hundred training rounds being fired to proof-test a bolt and screen out infant mortality. Testing each bolt with several hundred rounds would be cost prohibative, so it is somewhat understandable that the DOD would demand MPI on every bolt.
It is my opinion that once a bolt has experienced several hundred rounds, the benefit of MPI is gone. If there were a defect in the bolt, several hundred rounds of fire would reveal it with more certainty than MPI screening would.
I am not trying to convince anyone that they should not buy a Mil-Spec bolt. However, if you have a non-Mil-Spec bolt that has several hundred rounds through it, you have essentially screened it for manufacturing flaws, which is all the MPI is supposed to do.
An AR-15 bolt is made from high strength hardened steel, current Mil-Spec calls for Carpenter 158 (originally a steel for molding plastic). Due to the nature of this class of steel and the high operating stresses these parts are designed for, a flaw at a high stress (or high Kt) area will quickly grow into a crack, and that crack will propagate through the part until it reaches a low stress area, or until the part fractures. The complex geometry of an AR-15 bolt makes fracture more likely. Bottom line, if a bolt has an initial flaw, failure of the bolt will probably occur sometime between the first few hundred rounds.
A bolt that is purchased by the DOD may very well be used in combat from the very start of its life. The bolt could be used in a new weapon issued to a soldier or marine in a theatre of operations, or it could be a replacement bolt used to repair a weapon. Either way, the DOD can not count on several hundred training rounds being fired to proof-test a bolt and screen out infant mortality. Testing each bolt with several hundred rounds would be cost prohibative, so it is somewhat understandable that the DOD would demand MPI on every bolt.
It is my opinion that once a bolt has experienced several hundred rounds, the benefit of MPI is gone. If there were a defect in the bolt, several hundred rounds of fire would reveal it with more certainty than MPI screening would.
I am not trying to convince anyone that they should not buy a Mil-Spec bolt. However, if you have a non-Mil-Spec bolt that has several hundred rounds through it, you have essentially screened it for manufacturing flaws, which is all the MPI is supposed to do.
