Forgive me I'm gonna ask something dumb about AR building or piecing together

Ar15 and Ar10s are COMPLETELY different animals.

With an AR15, assuming a complete upper with bcg, then yes two pins on your complete lower, load mag and shoot.

With ar15s its possible to mix and match multiple brands almost at will, you will sometimes run into a slight fitment isdue usually due to production tolerances.

With ar10s there are 3 to 4 different types that may or may NOT play nice together.

There are a lot of proprietary stuff within tbe ar10 market, its best to stay with one company or at least one particular pattern all the way through

I don’t have the time to get into this as I have to get ready for work. Someone else will come along with the differences between the lowers and compatibility. But I’ll advise you to stay away from the Beowulf unless you reload. Even then, it’s my opinion that the Beowulf isn’t that impressive ballistically. I did a lot of research before I chose my big bore AR’s. I took price, availability of commercial ammunition, ballistics, and supported components into account. I ended up on the 450BM. Others chose the 458 SoCom. Neither is a bad choice. But find me 150rnds of 50 Beowulf for $130 and I’ll eat crow. But that’s what I paid, plus the ammo can and shipping for my 450 ammo. Total with can and shipping was $159.25.

Yes, everything interchangable and pretty standard on the AR15 platform....except, that Colt lowers and uppers are LARGE pin, while almost all other manufacturers are SMALL pin. You have to stay with a pin size to be interchangable.

For AR10's, what newfalguy101 said is correct. I found that out when I bought a Stag 10 stripped lower, intending to build it and buy a cheap upper later, and then realized I was pretty much committed to having to buy a Stag upper. And you have to watch rail heights and such.

BigBore44 is also correct in that there are some calibers on the AR15 platform that you probably want to reload if you're going to shoot much of them. 223, 300 Blackout, 7.62X39 are all mostly good to switch and cheap ammo available, but 50 Beowolf, 450 Bushmaster, 458 Socom are all a bit pricey.

And you should know that there are 22LR dedicated uppers or conversion kits between a 223 upper and 22lr, and you want a dedicated upper, IMO. And AR15's also can be adapted for 9mm, but that's a little tricky and you shouldn't go that direction for your first AR.

lionking said:

Am I understanding correct that if you get a lower you can switch uppers on it just attach and shoot? Say I got a PSA lower for example I can add a .300 blackout upper, or 7.62x39mm upper and so on just attach and shoot away after getting other magazines? Or do things need to be adjusted on the lower first when using a different upper and or caliber?

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In general, you have it correct. Put together an AR15 lower with your preferred trigger group and stock, and you can slap on pretty muc any AR15 upper. There are some caliber-specific parts, such as bolts and magazines, but as a general statement, any "standard" lower should work on any "standard" upper.

I have dedicated lowers for all of my uppers, but I will often move them around if I want to switch to a different stock for example. It's quicker and easier than pulling the buffer tube and swapping out stock assemblies.

While Berettaprofessor's comment about the Colt large-pin receivers is correct, Colt switched to the industry standard pin size at least a decade ago, so unless you are buying an older gun, you should be good to go there as well. Actually, the old Colt SP1 guns are becoming a bit of a collector's item in some areas. An unfired Colt Sporter II is the only complete AR15 rifle I've ever owned. The rest have all been assembled from various parts. And the Colt had the odd takedown pin and fire control pins, as well as the worst, grittiest trigger I've ever experienced on an AR, so I ended up selling it to someone in New York, where a "pre-ban" rifle still meant something at the time.

I have paid less attention to the AR10 platform. A few different companies were developing their own commercial versions, so for a time there was some difference in magazine compatibility between lowers. I think they have all pretty well standardized now, but you would want to verify.

In any event, there's lots of info out there. Have fun deciding what you want to build. Just be careful, they tend to multiply.

GarrettJ said:

I have paid less attention to the AR10 platform. A few different companies were developing their own commercial versions, so for a time there was some difference in magazine compatibility between lowers. I think they have all pretty well standardized now, but you would want to verify.

In any event, there's lots of info out there. Have fun deciding what you want to build. Just be careful, they tend to multiply.

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While the ar10 industry has largely settled on the DPMS pattern, or at least much of it, there is still several companies with proprietary components, including Anderson and PSA.

A common path is purchase a complete AR15.
Next purchase a complete lower (must be shipped to an FFL if you order online) and a complete upper and snap them together.
Change some components on one of the above.
Purchase a stripped lower (if it’s not an 80% it has to be shipped to an FFL if ordered 9nline) and the parts to make a lower and assemble.
Purchase the parts for the upper and assemble.

Along the way you’ll need to pick up a few tools but AR15s are called adult Legos as they’re so easy to mix and match.

Otherwise all that was said above is true.

Don't be afraid to attempt a build of a stripped lower.

I’ve become a fan of Aero uppers and lowers for AR-15’s and DPMS pattern LFAR’s (“LR-308 pattern).

The DPMS pattern LFAR is really the way to go for a home builder in 2019, as the component availability and interchangeability is much greater than with the Armalite, GII, Mega, Stag, Bill’s AR shop, etc.

Complete lower... PSA, or check locally for a stripped lower, compare to online with shipping and transfer, I am fortunate to have a LGS that is competitive on lowers, then you need a Lower Parts Kit that can go from 20$ to hundreds depending on what is in it, I purchased the LPKs with a stock for 60$ to 80$ and smooth out the trigger.
Complete uppers... PSA or Bear Creek Arsenal, just watch and make sure it includes the Bolt Control Group if not they run 60$ and up.
Once you get a lower up and running you can install any of the newer uppers, I have a Colt purchased used in the mid 70s that doesn't work with new lowers. I have swapped out uppers for my grandson to shoot with different stocks.
A friend has a 9mm that uses Glock mags that is fun to shoot at closer distances.
They are fun range toys and can also be good for home defense and hunting

Sistema1927 said:

Don't be afraid to attempt a build of a stripped lower.

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While many here seem to be machinists, or have mad skills, I just have the abilities of an average homeowner. Yet I was able to assemble a stripped lower with just the tools you would find in an average home. I watched a few videos, and proceeded to assemble while starting and stopping a video as I progressed.

Took less than an hour, managed not to scratch or damage anything, and it worked perfectly from the first shot forward. Found I'm not really into them, sold mine, but if a great deal involves a stripped lower, you probably shouldn't let that stop you. For the record, mine was a $40 Anderson lower, the rest was an inexpensive PSA kit, with a pre-assembled upper.

Good luck with yours!

Side note: I have never seen the appeal of a complete lower. Thee of the five interface components between the shooter and the rifle are within the complete lower, and in building many AR’s for many people, I rarely find that all 3 of those components will suit a broad spectrum of folks - for budget, aesthetics, and ergonomics.

Fair to say, a new AR shooter might not know what they don’t know, so they won’t know what they want - which is why I generally say folks should either test drive a lot of AR’s first, working with a builder, or buy a standard 20” model used, planning to resell later once they are familiar with the platform and identify their wants and needs.

What we usually see as complete lowers are completely generic, Mil-spec or Magpul furniture lowers - if it fits you, great, but personally, I rarely see multiple customers with the same desires for stock, grip, and trigger.

I only buy complete lowers when either the price works out such “buying twice” is cheaper than buying once, or when a customer insists on exactly those components, and the price is it’s lowest opportunity. The vast majority of my AR builds have started on a stripped lower.

I’m surprised no one has mentioned several things here. Gas. There are 4 main gas lengths; pistol, carbine, midlength, and rifle. That’s ignoring direct (no gas), and a few intermediate lengths. What you choose could mean some changes to your lower, specifically the receiver extension (pistol, carbine, or rifle). That will in turn determine spring length and buffer weight.

Second, and a question I had when starting out, is what makes a pistol a pistol or a rifle a rifle. Building out of order puts you on the wrong side of the ATF, even having certain parts on hand without other ARs to attach them to may run you afoul of the rules.

These aren’t impossible issues nor are they overly confusing with a bit of reading, but they should be understood before proceeding.

I also agree with starting with a plan for components to avoid wasting time or money on things you don’t want. I’ve been there and done that with painful results when trying to make chicken salad out of chicken scratch. The flip side is that I now know what I like, what I cannot tolerate, and my limits on risk when it comes to new products.


Sometimes you stick to the plan, buying components on sale as you go and checking the boxes...



...other times new info modifies the plan.


Hopefully you get what you want in the end.

Skylerbone said:

I’m surprised no one has mentioned several things here. Gas. There are 4 main gas lengths; pistol, carbine, midlength, and rifle. That’s ignoring direct (no gas), and a few intermediate lengths. What you choose could mean some changes to your lower, specifically the receiver extension (pistol, carbine, or rifle). That will in turn determine spring length and buffer weight.

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Length of gas system has nothing to do with buffer & action spring choices for a standard frame AR. It doesn't matter if an upper has a pistol, carbine, middy, intermediate or rifle length gas system, it will function quite happily on a lower with a carbine, A5 or rifle RE assembly. A blow back AR (such as a 9mm) usually needs a heavier than normal buffer. The simple truth is, the sweet spot for buffers regardless of upper gas length is H2, A5H2, A5H3 and Rifle. These buffers combined with the appropriate spring (H2, standard carbine or Sprinco blue. A5, Sprinco green. Rifle, standard rifle or Sprinco green) gives a good combination of felt recoil, recoil control and carrier speed. (Uppers must be gassed correctly. If the upper isn't gassed correctly, different buffer weights and spring won't fix the problem.)

As you are a thoughtful student of the technical details of the AR, I figure you'll find this information useful.

@MistWolf much better (and correctly) put. I know they will function, I have 2 rifle gas and a 5.56 pistol that all run on carbine extensions, I was thinking simpler I guess or probably confused, but meaning to say a rifle buffer and spring don’t mix with a pistol tube/other parts may need swapped.

I believe it was you who explained to a few naysayers that a carbine extension did not make a pistol an SBR and pointed to the SBA3 as proof (for the OP it’s a pistol brace that fits a standard milspec carbine extension), which I later purchased for my build.

An aside; on your advice I also kept the fsb on my RRA middy and as you mentioned .085” being large for the port, swapped the carbine for an H2 buffer and swapped in a full profile carrier- another thank you. Always trying to learn, sometimes succeeding, may need another decade.

lionking said:

So if I was to get a lower what would you all recommend?

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Honestly, get whichever one that has the logo most to your liking. Or the type of finish you prefer. Or the least expensive. Seriously.

There really aren’t any “bad” lowers being made these days. And that’s likely because there are only 3 or 4 companies actually producing them. They get a marking variance from ATF and make them for all of the other “manufacturers”.

You used to hear about out-of-spec receivers from SGW/Olympic, and Sendra, and the garbage put out by Hesse. But none of them are around any more. And Olympic got their stuff figured out a couple decades ago, before closing shop more recently.

So get what you want. Put a good trigger in it, install your preferred style of buttstock, and slap on your favorite upper.

Honestly, you can swap uppers around, but for as inexpensive as parts are these days, you can set up a dedicated lower for every upper you own.

These are the Good Ol’ Days!

GarrettJ said:

Honestly, you can swap uppers around, but for as inexpensive as parts are these days, you can set up a dedicated lower for every upper you own.

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It is a little known, but true fact, that the universe is seriously out of whack if lowers don't equal uppers. As a result, it is a solemn obligation to build a lower for every upper you might get.

Sistema1927 said:

It is a little known, but true fact, that the universe is seriously out of whack if lowers don't equal uppers. As a result, it is a solemn obligation to build a lower for every upper you might get.

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Is that why lowers seem to spawn when there are extra uppers around?

Sistema1927 said:

It is a little known, but true fact, that the universe is seriously out of whack if lowers don't equal uppers. As a result, it is a solemn obligation to build a lower for every upper you might get.

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Nah. Shoes and socks.