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Discussion Starter · #1 ·
Hi, I'm about to buy my first ar15 bolt carrier group. It is really hard to find one if any right now and I have found one in stock and would like to know why the carrier is 8620 and the bolt is Carpenter 158-Case hardened. Isn't the carrier suppose to be Carpenter 158-Case hardened as well to be mil spec?

Mil-Spec MIL-STD-171 Phosphate Finish
Individually MPI Tested & Stamped
Bolt: Carpenter 158-Case hardened, Shot Peened, and MPI tested.
Carrier: 8620 Tool Steel-(M16 full shroud)-Heat Treated, Case Hardened, & Chrome Lined.
Gas Key: Tool Steel-Attached using Grade 8 Hardened Fasteners, & Staked.
Cam Pin: 4140 Steel
Extractor:Tool steel-shot peened
Extractor Spring:Upgraded Chrome Silicon. Black Buffer & O-Ring for Enhanced Reliability.

Can anyone let me know if this is a good bolt to buy or should I keep looking? Also price is only $119.95.


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metallurgy is always intersting stuff

Each material have very similar attributes. How they get there is very different. The 8620 has high carbon content and gains it strength through heat treating. The 158 carpenters steel has a high alloy content that gives it strength. The link below isn't the most user friendly but if you look closely you can find the material properties and attributes pages for those or any metals. Either one is a good choice especially at that price!
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