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Posted: 1/30/2006 7:34:31 PM EDT
[Last Edit: 1/31/2006 7:31:32 AM EDT by SamColt]
DPMS uses 7075 T6 Aluminum in line with industry standards - confirmed.
Disregard my original post unless you wish to talk about different aluminums etc.

well - I'm a research kind of guy and I have been snooping again.

Looking at Brownells they list a DPMS lower as 6061 Aluminum

They list a Rock River as 7075-T6 so I google 7075 vs 6061 aluminum and I learn the following.

Google's first hit - an RC car parts manufacturer

7075 vs 6061 Comparison Chart:
(T) Tensile Strength
(y) Yield Strength

Brinell Hardness
6061-T6

(T) 45,000 PSI

(Y) 40,000 PSI

7075-T6

(T) 83,000 PSI

(Y) 73,000 PSI

Metallurgists, What does this actually mean to us? A 6061 lower will crack after 20K rds and 7075 will last 40K rds? Sometimes my researh comes after I have spent my money I just finished a 20" A3 build using a DPMS lower and have another one in the works. I dont plan on changing anything out but I would just like to know from a practical standpoint what this means. The bolt lugs and barrel extension take the loading correct? Where would a polymer lower fit into this equation?

The only caveat is that I haven't confirmed that the Brownell's description is correct. I have found other errors in their product descriptions in the past....
Link Posted: 1/30/2006 8:15:39 PM EDT
[Last Edit: 1/30/2006 8:16:20 PM EDT by SamColt]
OK:

From the DPMS website below - I'm going to mail DPMS - I'll post their response if anybody is interested.

Upper Receiver:
• Forged 7075-T6 aircraft aluminum alloy
• Hard coat anodized per Mil Spec and Teflon coated black

Lower Receiver:
• Forged 7075-T6 aircraft aluminum alloy
• Hard coat anodized per Mil Spec and Teflon coated black
Link Posted: 1/30/2006 8:24:18 PM EDT
The industry standard (might even be mil-spec) is forged aluminum in the 7075-T6 variety, IIRC.

Chances are, Brownells just got it wrong.

WIZZO
Link Posted: 1/30/2006 8:28:10 PM EDT
I know Wizzo - I feel bad that I posted this first before checking DPMS' web site. I sent DPMS an email that Brownell's wasn't doing them any favors - just as an FYI.

SC
Link Posted: 1/30/2006 8:32:50 PM EDT
[Last Edit: 1/30/2006 8:33:05 PM EDT by Hooligan22]
It's hard to quantify how much better the 7075 is in this application, but it's just that, better. It's often the preferred aluminum alloy in fabrication (except where welding is necessary as it's difficult to weld). It's also usually four times the price of 6061!
Link Posted: 1/30/2006 9:18:47 PM EDT
I brought this issue up a few months ago. I was going to use it to slam DPMS but it appears there is a typo in the Brownells catalog.
Link Posted: 1/30/2006 9:55:11 PM EDT
7075 is aircraft grade that's all I know... but I wouldn't fly in a plane made out of stripped lowers
Link Posted: 1/31/2006 3:09:06 AM EDT
but you can verry well be flying a plane made of old beer cans. Most metals anymore are recylced material from something else.
Link Posted: 1/31/2006 4:53:51 AM EDT

Originally Posted By scottryan:
I brought this issue up a few months ago. I was going to use it to slam DPMS but it appears there is a typo in the Brownells catalog.



Yeah me too - I had mixed emotions, it pissed me off at first because when I bought my lowers I was told they were 100% mil spec etc etc yada yada - then I read the Brownells add. I was going to rip them on ARFCOM but I thought twice and said well hell some companies make lowers out of plastic why not 6061?!?! So I posted the above and should have gone a step further and found out directly from DPMS.....oh well - I'll post their reply today....

BTW: the 2 lowers I picked appear to be very well made. I was very pleased with the fit and finish - mated them with CMT M4 feed ramped "flat topped" uppers.

SC
Link Posted: 1/31/2006 5:29:20 AM EDT

Originally Posted By JBravo223:
7075 is aircraft grade that's all I know... but I wouldn't fly in a plane made out of stripped lowers


Negative, 6061 is also aircraft grade. 7075 is used for very high stress parts, such as landing gear struts, engine mounts, and the like. About 95% of an aircraft's structure (spars, ribs, stringers, bulkheads, and skin is 6061. Non structural parts are virtuallu 100% 6061 if they are made of aluminum.

I worked as a mfg support engineer for nearly 9 years at a major business/private aircraft manufacturer.

That said, there are significant differences in tensile and yield strength between 7075 and 6061, unlike the 4140/4150 steel issue, where the difference is very narrow.
Link Posted: 1/31/2006 6:31:27 AM EDT
AR-15 receivers are not supposed to be made from 6061. Colt's went ahead and used it for years, and finally suspended it's use in 1968. I have seen many XM16E1 receivers made of 6061 that are heavily pitted from corrosion.

There really ain't nobody still using that stuff on AR receivers, is there?
Link Posted: 1/31/2006 6:44:46 AM EDT
Hello,

6061 is NOT A STRUCTURAL aluminum alloy (at least in the commercial aircraft industry).

7075 is a used all over the structure of commercial airliners.

If your AR is made of 6061, it is certainly NOT as structurally sound as one of 7075!!!

7075 is used in fuselages and wings.
6061 is used for things like vent intakes and cabin fixtures.

One cool feature of 6061 aluminum is it can be welded fairly well for certain applications.

$0.02,
AE
Link Posted: 1/31/2006 6:45:33 AM EDT
Link Posted: 1/31/2006 6:56:58 AM EDT
6061 will almost "flake" apart during the corrosion process in the right enviroments. Wet, warm, galvanic, and so on...

7075 vs 6061 for cost... come on folks, do you know how little the actual price of material is? Even exotic stuff like Maraging steel - used in bolts, breech blocks, etc - is about $30 a pound. Regular steel is a matter of cents per pound.

I don't buy the it's too expensive line.
Link Posted: 1/31/2006 7:19:51 AM EDT
We've established it is a non-issue, b/c DPMS does in fact use 7075. But in response to the cost question, here are some prices from a common online metal peddler:

One foot length of 1" extruded round rod:
6061-T6: $4.39
7075-T651: $10.70

So rougly 2 1/2 times the price, at these quantities. At the quantities purchased by major manufacturers, who knows what the differences are.
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