Determining AR lower receiver quality

Discussion in 'Maintenance & Gunsmithing' started by AMproducts, Aug 3, 2012.

  1. AMproducts

    AMproducts
    Socal/Roswell, GA
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    One topic everyone discusses to death is quality of AR lowers, while I would give an ordered list of quality lowers, there are just too damn many of them. But here are a few pro-tips from someone in the industry (myself).

    1) Mil-Spec: There really are no mil-spec AR-15 lowers, because the military doesn't buy semi-auto AR's, none of them will be made to mil spec, so if someone says a part is "mil spec" you should be able to look up and cross reference which specs it complies with. Examples: If the anodizing is Type 3 hard-coat sulfuric acid anodizing, then it could be compliant with MIL-A-8625. Steel parts of the gun typically conform to MIL-STD-171 which covers manganese and zinc phosphate coatings on steel parts for corrosion resistance.

    The parts of the AR-15 that will never be mil-spec without an NFA tax stamp: Lower Receiver, Bolt-Carrier, and Barrel (must be 16"). If you are producing something to look like an M4.

    In general, nearly every aluminum lower starts life as a drop-forged chunk of aluminum made by 3-4 companies in the US, all of them are made out of 7075 aircraft aluminum and are heat treated to a T6 or better hardness. Plastic lowers and cast lowers usually do not conform to the normal material specs either because they're plastic (no mil-spec here) or because 7075 is a poor alloy for casting. Billet lowers may be made out of 7075, but there really is no requirement to do so in this case, as anyone with a CNC shop can start making billet lowers out of any of the commonly available aluminum alloys. (6061 T6 is a much cheaper, and still fairly strong alloy, but it is nowhere near as tough as 7075)

    2) Fit and finish - One thing I have been seeing a lot more of lately is very poor fit and finish on AR lowers put out by a number of companies, the principle offenders I've seen are Colt, Del-Ton, Aero Machine among many others. If you are thinking about buying one of these lowers (or the complete gun) run your fingers along the edges, especially along the back strap just above the hand grip and inside the trigger guard, if you feel a thick irregular mold line, make sure you're not paying a premium price for this lower/gun. What this means is in the rush to produce the lower, they couldn't be bothered to run an end-mill along the edge of the lower and cut that mold line off, this line is left from when the lower was hot-forged where the mold lines met, and extra aluminum was expelled. If you look at an S&W M&P15, they do a very good job of making sure this mold line is removed from the lower.

    Just for reference, some manufacturers do different things with different models, some of these are good, some are bad, some are indifferent. Getting back to the mil-spec point. Some of the newer "billet" lowers have dropped things like having the trigger guard as a separate piece, so forget about putting your own after-market part on there. The S&W M&P15-sport is a key example of this. However, the normal M&P15 is as close as you are going to get to an M4 without NFA taxes, it's a very high quality product.

    I'll try to keep this list up to date.
     
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  2. Bladegunner

    Bladegunner
    Sincity
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    Good info, Stay away from the New Frontier Armory Lowers There have been numerous reported problems of mag release buttons not fitting holes for pins not lining up especially in the "MS" run of lowers.
     
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  3. Joe Link

    Joe Link
    Portland, OR
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    Great post! I was particularly curious about 7075 vs 6061.

    It'd be really cool if you did a series on this with separate threads, "Determining AR lower parts kit quality", "Determining AR bolt carrier group quality", etc. With all of the manufacturers on the scene now it's getting harder and harder to know what is quality and what is crap.
     
  4. AMproducts

    AMproducts
    Socal/Roswell, GA
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    Holy thread revival!

    So the big difference between 6061 and 7075 is the alloying contents. 7075 contains Zinc, which makes it much tougher, and also more corrosion resistant. Don't get me wrong, 6061 is still a very good alloy, but it won't stand up to certain things as well, such as salt spray, and generally a 6061 part will bend where a 7075 part will break (fracture) However this is only in the case of the part being used well over it's designed load. (normally you would make a 6061 part thicker) While less of an issue in receiver parts, there's the issue of thermal creep, 7075 will resist this much greater than 6061, but this is more an issue for things that are run at 700F for long periods of time.

    TBH, I haven't spent much time finger-f'ing the guns at gun stores much lately, and I'm bored enough with the AR platform that I don't even bother looking at them when I go to SHOT. I did check out the CZ 801 Bren at SHOT this year (I was kinda thinking about it) however I really don't like the ergos on it, despite my size I considered the length of pull a bit long, and the mag-well frame was really long for what it was (I think this was so they could have a .308 version on the same receiver/chassis. I also checked out the SCAR which I really like, however I've got other things to spend that kinda coin on at the moment.

    Plus, I'm also starting to work on some of my own designs. I'm working on a SCAR clone that uses commodity AR-15 parts where possible. Dropping the CNC'ed aluminum in favor of stamped or extruded parts. These days it's quite expensive to get into drop forging, and it's pretty much something you can forget about doing on the small scale. However you can deal with manufacturers all over the place that will custom extrude a shape out of pretty much any material you want for about $14k. (this wouldn't even pay for the drop forging die!)
     
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  5. Joe Link

    Joe Link
    Portland, OR
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    I didn't even notice the date!

    Good additional info there, thank you :)
     

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