AR types: Is Mil-Spec 4140 or 4150?

Rifleman.308

December 02, 2002 06:45

I've been looking at Armalite's mid-length 16" setup lately, and asked Armalite directly why they machine from 4140 steel. I had understood (w/o any independent research, I must admit) that mil-spec for M16 rifles was 4150 steel for the barrel. I recall seeing a thread on ar15.com that confirmed this as well.

Armalite emailed me this a.m. and stated that 4140 is mil-spec because mil-spec looks only to whether the barrel rates 28-32 Rockwell, NOT to metallic content.

Anyone know the real skinny on this one?

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  hondo

  December 02, 2002 08:58

  The only difference, as you probably know, between SAE 4140 and 4150 steel is the Carbon percentage in the steel. Both are Cromium-Molybdenum steels. The Carbon percentage for 4140 is 0.38 to 0.43 while 4150 is 0.48 to 0.53. Therefore the dominate factor becomes the Rockwell number, 28 to 32..........Hondo

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  Rifleman.308

  December 02, 2002 09:15

  Yup, I was with you that far. My question was, if anyone knows, does Uncle Sam specify Rockwell hardness or actual metallic content in the formal specs? I don't really give a damn because I think the discussion is largely academic, particularly with a chrome-lined barrel, but mil-spec is mil-spec, if you know what I mean.
  
  Thanks, Hondo. Do you know the answer to this one?

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  select-fire

  December 02, 2002 09:56

  I can't answer Uncle Sams ratings on guns but in the aerospace industry they required Both metallic content and RC verification. And of course with any steel or alloy there are tolerances allowed to be in spec. That is why sometimes the same steel, alloy or aluminum will machine differently on each batch of material. That is also where the Rockwell factor comes in. To be sure the hardness is the same. That is also WHY parts fail, fractures can occur when the steel is heat treated to the rc number. Hopes this helps I did the inspection and certification on military and commercial aircraft before retiring. I always had to send a sample piece of steel to verify the parts were made from that same batch of steel.

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  hondo

  December 02, 2002 10:27

  Rifleman.308 I don't have a copy of the Mil-Spec for the M16. Like all Mil-Specs there will be a seperate Spec just for the barrel and typically will be many pages long. Iam sure however that it will contain both Rockwell Hardness limits and the 41XX series steels that are accetable. This doesn't answer your question directly but I will dig around and see what I can come up with............Hondo

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  cpermd

  December 02, 2002 11:45

  Mil spec is 4150 steel.
  4140 is minimally different and there would not be any difference in longevity if fired SA.The chromelining is more important to me.
  cpermd

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