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Old February 14, 2014, 09:53 AM   #1
hlds54
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Questions about AR uppers and Lowers

Soon I am going to build my first AR-15. I've decided to completely build the lower receiver and then buy an assembled upper. My question is are there Commercial uppers/lowers and mil spec upper/lowers? I understand there are different buffer tubes, but can i just throw any upper on any lower and it will fit(assuming they're the same caliber)? I'm not the most knowledgeable about ar-15s so someone please clarify/correct me.
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Old February 14, 2014, 11:20 AM   #2
Jay24bal
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Yep, as long they are both for the same caliber (not trying to put an AR-10 upper on an AR-15 lower or vice versa), they should fit. As you stated, the buffer tube is different between mil-spec size and commercial size, but all upper and lower receivers are the same.
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Old February 14, 2014, 11:23 AM   #3
hlds54
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Jay24bal Thank you so much! Answered my question perfectly.
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Old February 14, 2014, 05:28 PM   #4
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The upper and lower will fit together but there is a vast difference in quality when it comes to the parts. If you are buying a complete upper look at names like BCM, PSA (premium), Del-Ton (good quality reasonable price) and on to the higher priced units. The BCG and barrel are the heart and you should not skimp there, all the above brands are GTG. There are many more good makers/assemblers, I mentioned only a few that are reasonably priced but excellent quality.
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Old February 14, 2014, 09:29 PM   #5
Mobuck
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Caliber(within receiver size) has nothing to do with fit or function as long as it's not something strange like a pistol caliber upper. A 223 upper interchanges with a 300 AAC or a 5.45x39 or 7.62x39(with correct magazines).
"Mil-spec" and "commercial" only applies to the diameter of the buffer tube and butt stock on a collapsible stock.
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Old February 14, 2014, 10:40 PM   #6
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almost any upper will fit almost any lower. there are some that do not like to pair but just about any of the dozen or so name brands will mate to eachother without issue.

as for caliber, in ARs, most of the time the upper half is what controls caliber. all of my ARs say they are 5.56 or 223 wylde but in fact they are 9mm, 5.56 and 6.5 grendel. with the exception of some specialty calibers, the lower makes no difference at all when it comes to mating uppers.
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Old February 15, 2014, 10:48 AM   #7
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Here's a nice lower you might consider:



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Old February 15, 2014, 01:34 PM   #8
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Where was that lower when I built mine?!? Hahaa!!
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Old February 16, 2014, 09:35 AM   #9
hlds54
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Thanks everyone for the info. Will Come in handy when I build this baby. Which I'll be sure to put a thread about it. Really like that Brady campaign lower!
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Old February 16, 2014, 02:40 PM   #10
tahunua001
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forgive me if I'm wrong, but wasn't the brady campaign one of the most heinous, and restrictive gun control pitches ever to hit the senate and HOR?

I find it hard to believe that anyone would allow that name to be etched onto an evil black receiver.
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Old February 17, 2014, 12:52 AM   #11
marine6680
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Hence the joke...



As for the OP:

Any lower and upper that is made to mil-spec should fit together. Mil-spec gets thrown around a lot, almost to the point of worthlessness, but when it comes to receivers, it means that it should be built to certain dimensions and of certain materials. There are some cheap receivers that are built to mil-spec dimensions, but are made of non-mil-spec materials. Meaning standard parts will (should) fit, and other receivers should mate with it.

When it comes to materials, finish, and other areas, Mil-spec can be exceeded and improved upon. Where mil spec really matters is dimentions, because that ensures parts fitment/compatibility.

Mil-spec can also be looked at as a minimum standard to ensure a rifle that is of a known good quality. Like I said before, some rifles are made with lower cost materials so they can be sold at lower prices. These materials are usually weaker than the mil-spec materials. To some this matters. Though these rifles are usually fine for a low use rifle that gets fired little...

If it maters to you, then learn what materials are proper spec. Areas where the price skimping happens usually are... Receivers, Barrel, Buffer tube, Bolt carrier, and Bolt... The Bolt and Bolt carrier are the heart, and you don't want to skimp there, the barrel is also important. A non-spec material barrel is not automatically bad, if it is made well it can be accurate. It just might not hold up well to rapid fire or last as long before needing replaced.

Probably more info than you needed, but there you go.

Last edited by marine6680; February 17, 2014 at 12:57 AM.
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