courtney- question about the makeup of stripped lowers for ARs

[Courtney.cj.chen]

I just purchased two stripped lowers to build for my two little boys (future toys/life preservers) - I was looking for aluminum ones at reasonable prices... but.. I bought a polymer one bc I found one for $70 (yes a week ago), and bought a Generation 4, Alloy one from Mag Tactical for $150. Can someone please send me a link educating me, or their own response on: aluminum, alloy, and polymer "grades" (esp the first two- metal ones) - what i mean exactly is: i know oftentimes the aluminum that is used for these lowers is 7075-T5. Then i see 6066 T6 aluminum. and 8625. Which ones are the most durable, relatively speaking- and what does the T4-T6 etc. all stand for? and which are alloy. and lastly- are there different grades/qualities of polymer as well? And yes- I am looking for a "boring" chemistry class/gun smithing/etc-type of DETAILED explanation please! and thank you!
Courtney
p.s. Boring = Exciting (for me in this case)

 

[S&W645]

The first 4 digits give you the alloy. The T is the hardness treatment given to the alloy.
Data on 7075-T6 is at ASM Material Data Sheet Don't know anyone doing a T5 to it. The 7075 is the stronger alloy and used in forgings while 6066 is an extruded alloy normally. 6061 which is close is at ASM Material Data Sheet

7075 is about 70% stronger than 6066.

 

[BluesStringer]

Wow. That's a huge project to dive into X's 2. Depending on your access to machining tools, or better, access to a machine shop, it is really going to be a challenge, especially on the aluminum 80% lower. I have a friend who is a fairly large manufacturer of aluminum lowers (tacticalmachining.com in FL) and I've been considering starting with one of his 80%'ers for a bottom-up build like that for several years. I've got 20 years in the steel industry (not aluminum, but similar disciplines), and I haven't taken on that kind of challenge because I don't have the right tools, or local friends who do, or access to a machine shop etc. If you have that kind of help, more power to ya, it's still a daunting task for a beginner. If not though, you may well have bitten off more than you can chew. I wish you luck either way, and admire your ambition too. Seriously, good luck.

On the number designations, they derive from the American National Standards Institute (ANSI) in conjunction with The Aluminum Association. ANSI covers national standards for thousands of different materials, so to get the kind of detailed info you're looking for, you'll probably have an easier time of it going to TAA website, since they deal only with aluminum.

Basically though, to start, all aluminum is "alloy" regardless of the numbers that further detail its different compositions, tempering and tensile and shear strengths. I can't tell you exactly what each column of numbers refers to, but I can tell you that those are the kinds of things they designate. All aluminum alloys have four-number designations, and the first number tells you what the main mixture of different metals and elements is. Here's a chart showing those basic main components, but as you can see, the first number doesn't tell you how much of any one metal or element is present (except for 1000 series, but I've never seen that used for anything other than ordering raw material to cast or roll into an alloyed composition). That is detailed by the other three numbers.

Series	Main Alloying Elements
1000	Pure Aluminum (99% min.)
2000	Copper
3000	Manganese
4000	Silicon
5000	Magnesium
6000	Magnesium & Silicon
7000	Zinc
8000	Lithium & Other Additive Elements

If you go to TAA website and search on the specific number designations you mentioned, you can probably find the meanings of the remaining three columns of numbers pretty easily. If the specific numbers don't get you adequate results, try "properties" or "composition."

I'm guessing the "T" designation refers to temper or hardness. I can tell you how that measurement relates to why you might choose one hardness rating over another. Let's say (hypothetically) that a specification calls for an aluminum column within a building of T3 tempering, while calling for an aluminum gusset of T6 tempering. First of all, understand that I could be mistaken that that's what the "T" refers to to begin with, but I'm just trying to tell you what purpose is served by making any kind of structural component harder or softer. So in this hypothetical, the column is softer (T3) and the gusset is harder (T6). Look at this elementary illustration of the two structural members I'm referencing:

The column is the vertical piece, and the gusset is the triangular piece tying in the vertical column with the horizontal beam as a reinforcement of the joint. When a piece of metal is "soft," it will bend more without losing its structural properties than a piece that is "hard." So the column in our hypotheitcal building is softer so that it can bend a little in the wind. When the wind dies down, the column will return to its original position with no cracks, tears or shearing from the stresses imposed by bending it. The gusset on the other hand, doesn't have to bend. It is there to make the most secure joint possible. The stresses imposed on it are compression and/or stretching as the other members do the bending, so giving it a high temper increases its resistance to the kinds of stresses it will encounter within the assembly. If the column or beam were tempered as hard, they would be brittle and susceptible to failure under bending stresses.

So now let's relate those kinds of properties to building a firearm. Your alloy lower will have almost no bending stresses put on it, and if I'm right that "T" stands for temper, a T5 or T6 rating is a pretty high (hard) rating. Makes sense. Too soft and it could become easily deformed when dropped on a rock in the field for example. Too hard, and it could crack from being brittle when dropped on the same rock. Firearms, especially semi-autos that fire rifle rounds, take a constant beating under use. In short, the alloy designations are selected for a rifle according to the kinds of stresses they will most likely be exposed to under normal use.

Hope that helps. I taught welding and was a construction estimator for a combined 10 of my 20 years in the industry. I had to teach guys (and a few gals) how to weld on aluminum and other alloys, but my seven years as an estimator dealt almost exclusively with steel structures, thus my going straight to a column and beam structure for an example. If you have any trouble finding the rest of the numbers' meanings within the designations, let me know and I'll see if I can track 'em down for you. You don't really need them, but I understand the "I just want to know" meme. I suffer greatly from that affliction myself. ;-)~

Blues

 

[Courtney.cj.chen]

Hey Blue,

Based on your answer- I realized that I mis-wrote my question- because the content of your most informative response- was giving me far too much credit. Where I wrote "stripped lowers to build" I should've written "stripped lowers to build an ar-15" so there will be no welding. I would be simply purchasing a lower parts kit from a good company- But yes- in past several months of looking for stripped lowers- I came to seek the 7075-T6 (T5 was a mistake) as a basic standard- and have really began to be curious in getting pointed towards some reading so that i could better understand these "grades" (<- is that what they are called?) in my everlasting "want to know" affliction.

It was a certain situation that caused me to post this thread: I was at my LGS looking for my 2nd stripped lower. and I picked up the G4 (generation 4) mag tactical multi cal stripped lower. It was incredibly light- and my only experience in something that was that light, was polymer- but it felt slightly denser- the guy informed me that it was in fact aluminum. And I asked him what "grade" aluminum it was, such as how i was looking for 7075, and "what type was this" because I needed to MAKE SURE that i wasn't paying a metal price ($150) for what might be a high quality polymer. But he said he did not know- but assured me it was "some" metal. I looked up the company online and it was alloy- and right after my purchase- since the guys at the LGS were familiar with me and my toddler son- they teased that I asked too many technical questions- but this only furthered my need to read up on this material. And after posting this earlier thread- I did read up on billeted and forged lowers- and based on the mixture of responses i found everywhere- I would love to know which you feel is a stronger method. If i'm not mistaken- billeted would be like taking a cube of marble and creating a statue from that piece & is the costlier method?, whereas forged would be like a sword that was then hammered to a certain shape/edge?? and how do i know which lowers were forged/billeted?

Thanks for another very educating post.
Courtney

 

[beachbum17]

Lot of parts on the AR are polymer more and more try days and lowers are moving more in those directions. Yes aluminum is more desirable, but I too but a polymer (omni tactical) for $110 for future build and all my friends got one too. They are mil spec as should do their job fine. The place where they will take more stress is the threading on the buffer tube, other than that the stress shouldn't be too hard them. Will work and do their job.
Lower parts kits can be ordered of rock river arms still, but those aren't going anywhere.

With the polymer lower I'm planing on doing a huldra or Spikes 5.45 x39 upper so I can shoot cheap Russian ammo and with the polymer lower it won't rust or need a good cleaning very often.

 

[S&W645]

Forged and billet can be one in the same but billet does not always mean forged. A steel billet is rolled to form an I-beam but can also be forged to form the same shape. Just as an aluminum billet can be forged to make a lower. Or it can be machined out of the solid billet. Forging metal aligns the molecular bonds better in most cases making a stronger product. It is very hard to make a casting that matches a forging in strength in steel. And cast parts tend to be weaker due to grain of the metal not being as aligned.

Raw Al or Fe poured from a ladle/pot/crucible to form a cast billet. Billet can then be rolled or hammered to a different shape. The new billet is then formed by machining to make a lower and described as a billet lower. Or it can be hammered to form to make a lower and it is a forged lower. You can also form a forged lower by starting with a casting and hammering it to shape. Forging always involves hammering either by hand or by a machine.

 

[Courtney.cj.chen]

With the polymer lower I'm planing on doing a huldra or Spikes 5.45 x39 upper so I can shoot cheap Russian ammo and with the polymer lower it won't rust or need a good cleaning very often.

I am putting a .22 upper on the polymer lower (also from Omni- i got very lucky and got one from palmetto right when they restocked- for $70+shipping) and 5.56Nato on the alloy lower. When you speak of wear on the threaded area for the buffer tube- what do you feel would be the worse that could happen- cracked? change shape- and/or no longer able to contain the buttstock. Do you think the OMNI lower can withstand 7.62Nato's? The 22 upper will be switched out later when my kids get older. still haven't decided if i'm going with a complete upper or a Stag arms bolt carrier group conversion kit in terms of the 22.

 

[BluesStringer]

Hey Blue,

Based on your answer- I realized that I mis-wrote my question- because the content of your most informative response- was giving me far too much credit. Where I wrote "stripped lowers to build" I should've written "stripped lowers to build an ar-15" so there will be no welding.

Ahh....no, that was my mistake, because you did write "stripped," and me being of an "80% lower" mindset, just thought that's what you meant. So you know, the 80%'ers don't require welding, just machining, but you don't really need to know that now that I understand what you're dealing with.

Just to make sure that I do understand what you're dealing with though, does the stripped lower have a serial number on it? I'm assuming it does. An 80% lower doesn't require a serial number, and generally speaking, that's the appeal for building one that way. The law allows anyone to build any otherwise legal weapon to buy in the retail market in their garage without having to register as a manufacturer with BATFE. Vendors that make major components (like lowers) can sell them as kit components as long as they're only 80% finished, thus allowing them to sell sans any serials, thus making them a preferred project for people like me who think the government will eventually come after our guns. They can't seize what they can't find, and they won't even know to look for it if there's no record of its sale, and the 80% lower keeps it all nice and legal.

I would be simply purchasing a lower parts kit from a good company- But yes- in past several months of looking for stripped lowers- I came to seek the 7075-T6 (T5 was a mistake) as a basic standard- and have really began to be curious in getting pointed towards some reading so that i could better understand these "grades" (<- is that what they are called?) in my everlasting "want to know" affliction.

For your purposes "grade" is fine. Industry-types would use words like "properties" or "composition," but grade works just fine for casual conversation.

It was a certain situation that caused me to post this thread: I was at my LGS looking for my 2nd stripped lower. and I picked up the G4 (generation 4) mag tactical multi cal stripped lower.

OK, now I know I get it. I did an image search using the bold text and see that indeed they do have serial numbers. So you had to fill out a 4473 NICS form to buy them, right? Even though it's a 100% machined piece and has the numbers, that's still a pretty impressive and challenging project.

And after posting this earlier thread- I did read up on billeted and forged lowers- and based on the mixture of responses i found everywhere- I would love to know which you feel is a stronger method. If i'm not mistaken- billeted would be like taking a cube of marble and creating a statue from that piece & is the costlier method?, whereas forged would be like a sword that was then hammered to a certain shape/edge?? and how do i know which lowers were forged/billeted?

Yeah, the statue/hammered sword analogy works well. Billet alloy is pretty much the gold standard of manufacturing processes. Like I said, I'm not well-schooled in alloys, but I've purchased hundreds of tons of it as an estimator, which didn't require purchasing billet parts, but I've also built motorcycles since I was 19 years old, and billet wheels, lower legs, triple trees, risers, primary and crank cases, valve covers, shifting and braking controls etc. etc. that are billet are always the most preferable to have, but they are significantly more expensive than their cast/extruded/forged alternatives, so I rarely use them in my builds just to stay on the frugal side of a semi-pro hobby. But yeah, billet is both stronger and costlier, so you pretty much got it down pat.

By the way, S&W645's answer was much better than my first one. Much more concise too! I took the "boring" part to heart though, so there ya go.

Blues

 

[Courtney.cj.chen]

....the 80%'ers don't require welding, just machining, but you don't really need to know that now that I understand what you're dealing with.

Just to make sure that I do understand what you're dealing with though, does the stripped lower have a serial number on it? I'm assuming it does. An 80% lower doesn't require a serial number, and generally speaking, that's the appeal for building one that way. The law allows anyone to build any otherwise legal weapon to buy in the retail market in their garage without having to register as a manufacturer with BATFE. Vendors that make major components (like lowers) can sell them as kit components as long as they're only 80% finished, thus allowing them to sell sans any serials, thus making them a preferred project for people like me who think the government will eventually come after our guns. They can't seize what they can't find, and they won't even know to look for it if there's no record of its sale, and the 80% lower keeps it all nice and legal.

Blues

Yes- I filled out the 4473 for my stripped lower (yes- it has the serial number on it)- and the polymer one i ordered online, as well- will be delivered to the FFL.
I just did some reading on the 80% lowers. Of course I had no idea of their existence prior to your last post. I will, 100% of the time, enjoy the "boring" explanation- so glad you did take it to heart-
Courtney

 

[S&W645]

Ahh....no, that was my mistake, because you did write "stripped," and me being of an "80% lower" mindset, just thought that's what you meant. So you know, the 80%'ers don't require welding, just machining, but you don't really need to know that now that I understand what you're dealing with.

Just to make sure that I do understand what you're dealing with though, does the stripped lower have a serial number on it? I'm assuming it does. An 80% lower doesn't require a serial number, and generally speaking, that's the appeal for building one that way. The law allows anyone to build any otherwise legal weapon to buy in the retail market in their garage without having to register as a manufacturer with BATFE. Vendors that make major components (like lowers) can sell them as kit components as long as they're only 80% finished, thus allowing them to sell sans any serials, thus making them a preferred project for people like me who think the government will eventually come after our guns. They can't seize what they can't find, and they won't even know to look for it if there's no record of its sale, and the 80% lower keeps it all nice and legal.



For your purposes "grade" is fine. Industry-types would use words like "properties" or "composition," but grade works just fine for casual conversation.



OK, now I know I get it. I did an image search using the bold text and see that indeed they do have serial numbers. So you had to fill out a 4473 NICS form to buy them, right? Even though it's a 100% machined piece and has the numbers, that's still a pretty impressive and challenging project.



Yeah, the statue/hammered sword analogy works well. Billet alloy is pretty much the gold standard of manufacturing processes. Like I said, I'm not well-schooled in alloys, but I've purchased hundreds of tons of it as an estimator, which didn't require purchasing billet parts, but I've also built motorcycles since I was 19 years old, and billet wheels, lower legs, triple trees, risers, primary and crank cases, valve covers, shifting and braking controls etc. etc. that are billet are always the most preferable to have, but they are significantly more expensive than their cast/extruded/forged alternatives, so I rarely use them in my builds just to stay on the frugal side of a semi-pro hobby. But yeah, billet is both stronger and costlier, so you pretty much got it down pat.

By the way, S&W645's answer was much better than my first one. Much more concise too! I took the "boring" part to heart though, so there ya go.

Blues

And here I enjoyed your answer better than mine. Must be the metal worker coming out in me.