When buying Mil-Spec aluminum parts such as a buffer tube, it is my understanding that one type of aluminium is considered mil-spec and the other is not.......its not just the size difference?

So is 6061 or 7075 true mil-spec?

7075

Just a suggestion, go with Colt, BCM, LMT or ALG.You wont regret it.

The size difference is Milspec vs. Commercial.  The grade of the aluminum is another matter.  I think Milspec calls for 7075.  However, I doubt you'll run into any trouble with 6061.  7075 has higher tensile and yield strength, and if tempered correctly, should not be any more brittle than 6061.  However, 6061 is cheaper, and for most uses, you'd never know or see any difference.  If you start looking at free float hand guards, it gets fairly difficult to find them in 7075.  Sure, they're out there, but the 6061 has proven to be extremely serviceable.

Anyway, 7075 is what you're looking for, but I wouldn't get worried if you can only get 6061 for now.

All my range beater, plinking, and training, and "budget build" rifles have 6061 tubes, mostly because I got great deals on them, and am a cheap bastard that likes to put rifles together as frugally as possible.

They have seen more use/abuse than my "serious use/shtf " rifles that I went out of my way to put 7075 tubes on.

There has never been an issue with the 6061 tubes. If it's a 6061 tube from a reputable mfg. it's GTG ..........at least for a range toy/plinker.

I think the only "potential" weakness in a 6061 tube is the occasional "digging in" of the end plate nub when torqueing the castle nut.  It only happens in a few cases, but it can happen.  The groove in the extension tube wants to turn with the castle nut just like happens with most nut-and-bolt interfaces.  7075 tubes pretty much guarantee that this won't happen.  If you grip the 6061 tube with a free hand, helper, or with some device while tightening the castle nut, the nub will not dig into the groove of the extension tube.  I think 6061 aluminum is plenty strong for extension tube material for most any situations.  7075 just makes it more bombproof.

I'm too tired to dig in the archives so..

An arfcommer tested 6061 and 7075 tubes. With sideways force applied via test rig and loading measured with in line tensiometer. The main difference was the 6061 tube bent more than the 7075 tube did before breaking. Both tubes broke within a few pounds of each other.

If my memory serves me correctly you seek 7075 T6 impact extruded aluminum and those manufacturers usually roll threads on.  Some companies use a different cut thread and offer a 6061 aluminum.  IM not an expert but this is what I have learned.

It goes like this.

Not specifically the answer, but be advised "mil-spec" is not a synonym for "the best". it means it conforms to the current at the time of manufacture specifications for military weapons, some of which date literally from 50 years ago, and cannot be changed just because there are vastly improved options. It's a labyrinthine bureaucratic process to get anything improved approved and the specifications changed. So don't get too caught up with the term. Technically, if it isn't made for the military on a military contract, it isn't really "mil-spec" anyway. Nothing on a civilian semi-auto AR-15 is actually mil-spec in the strict sense of the term, because Mil-spec only applies to the M16/M4 select fire weapons family. A more accurate term I just made up might be "Mil-spec style", kind of like a "a military style assault weapon", another meaningless term that is self-refuting.
Patrick Sweeney covers this issue in some detail in the Gun Digest Book of the AR 15.

https://www.amazon.com/dp/0873499476?tag=vglnk-c102-20

Not sure if it's the end plates I used, but the threads on my 6061 tubes get jacked up way easier than my 7075 tubes.

First, while you're shopping, pay attention to the wording.  If it says "mil spec size," then that's all they're claiming: the size.

I like "MIL-SPEC" receiver extensions (that's the formal name for what people call "buffer tubes") because I find more options for stocks in that size than in the commercial size.  

But I also like that, if they are truly MIL-SPEC parts, they're made from a tougher alloy that has specific treatment and finishing requirements.  

I doubt I'll ever have an opportunity to abuse a receiver extension to the point where I find out that the alloy and such make a difference, but I'm more comfortable knowing that I have that tougher part.

Just buy 7075 mil spec and be done with it.

A mil spec AR is almost identical to a Military version with the exception of about $50 worth of small parts.

The tricky part is in the wording.  Some pick and choose when to use the term "mil-spec."  Some shops make a 6061 extension with a "mil-spec" diameter.

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