Is there really any way of determining if a buffer tube is 6061 or 7075, other than the sellers say so?  I know 7075 is preferred, but I'd think 6061 should be fine unless you plan on butt-stroking trees - in which case both might prove inadequate.

The way it tastes of course..........

Jk, that's  a good question, kind of why I buy from known reputable dealers. I'd agree, 6061 is fine for a lot of peoples needs.

Cut off a small chunk and send it off to a lab for analization.

Sorry, but that one needed fixing...

maybe a handheld xray metal analyzer

Thank you I was struggling with that one

Yup, kinda what I thought.  I was just thinking there might be some little machining quirks between the metals that may make one distinguishable from the other.  The thought of going through the analization process was just to painful to consider.

Interesting

AFAIK there is no sure fire way other than having it tested.
We require certificates for some of the materials we buy at work.
We have some customers that require independent lab testing of the finished product.
For my AR parts I stick with suppliers that I know or ones that have good reputations for quality products.

6061 is approximately 4% LIGHTER than 7075.
A receiver extension weighs approximately 3-4oz .
A receiver extension should displace something like 35 mL of water.

All of these are simply measured quantities... so just determine the density of your buffer tube. A high quality quality large (1000mL) graduated cylinder, a scale capable of sub-gram precision, distilled water, and a pipette should give you a good idea of what kind of alloy your receiver extension (buffer tube) is.

If it's closer to 2.70 g/mL it could be 6061; if it's closer to 2.81 g/mL then it could be 7075.
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061T6
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

The forging (impact extrusion or whatever) process is going to completely screw up your density expectations. Forging (or impact extrusion and other similar processes) alter the density of the material.

Buy it from a reputable vendor.  A quality vendor will have quality sources that will supply exactly the product they say they are.

Yes, from a range commando perspective there should be no real difference in performance.  But a few things typically go along with the 7075 alloy: they’re almost guaranteed to be MIL-SPEC extensions.

MIL-SPEC receiver extensions are made to MIL-SPEC dimensions held to tight tolerances, they’re made with known production processes, they have high quality coatings, and rolled threads (which are tougher than cut threads sometimes found on lesser extensions).

So long story short, buy a MIL-SPEC receiver extension from a trusted source and you’ll get all the positives that go with MIL-SPEC, including the right alloy.

forging, while it can remove lattice deformations and microscopic cavities in a cast part, do NOT appreciably change the metal's crystalline density. The apparent specific density can change a tiny bit... but it's so small it's generally unaccounted for in design engineering.

I found no papers outlining evidence that aluminum parts are significantly more dense after forging. One would think, with the engineering implications of such a phenomenon, that there'd be tons of papers out there and charts describing density differences of forged vs extruded/cast aerospace aluminum. The only related paper I was able to find was a study based on carbon steel (cast ingots vs forged ingots) and the paper concluded that there was about a 0.64% difference in the density of a forged carbon steel. That ain't much.

Based on what I've found, I'm going to have to say... no, a forged RE tube wouldn't completely screw up the density expectations.