Does Aero Precision AR308s do anything different than the others?

Basic user · Oct 11, 2024

Decided on a collapsible (carbine) stock for this new Aero 6.5 gas gun build and the bolt won't go all the way back to lock open; it's about a quarter inch short. The buffer springs bottoming out and I can not figure out why. I know the whole "there's no single spec for ar308s/ar10s" thing, but finding out what's compatible and what's not seems to be pretty challenging. I keep seeing that the buffer tubes and buffer springs are compatible, but buffers are not. So, then how does this carrier tail bottom out? The tube's Strike Industries, the buffer's an adjustable Expo Arms, and the springs a no name AR15.
So during my research, the only thing I found was that on Aero's website, their 308 carbine spring has a different part number than their AR15 carbine spring... but when shopping on all the ar parts websites, they all list (except for aero) all the same springs for the 308s and 556s...
Called Brownells and they list the OAL's for the springs in their notes, and the 308 springs thicker and longer lol. Would Im imagining would exacerbate my issue (if those measurements are correct) or does the 308 spring have so fewer coils that it maintains a shorter compressed OAL...

Has anyone built one from the ground up on an Aero set and run into this same problem?

jetjock07 · Oct 11, 2024

There are two different systems, carbine and rifle configurations. You may have a rifle buffer spring in a carbine buffer tube.

Basic user · Oct 11, 2024

I took it out of my AR15 carbine. Does that make a difference? I'm thinking not.

jetjock07 · Oct 11, 2024

Just giving suggestion based on info man.

Basic user · Oct 11, 2024

It has to be this spring... I'm thinking Primary Arms' stuff is wrong lol.

Basic user · Oct 11, 2024

jetjock07 said:
Just giving suggestion based on info man.

No I get it; we have to try everything. I will get this sorted lol.

loclass · Oct 11, 2024

Are you using a standard ar15 buffer in a standard 7" buffer tube? If so you have the wrong buffer. If you're using a 7" carbine buffer tube on an LR308 then you need the "shorty" lr308 length buffer.

bfoosh006 · Oct 11, 2024

loclass said:
Are you using a standard ar15 buffer in a standard 7" buffer tube? If so you have the wrong buffer. If you're using a 7" carbine buffer tube on an LR308 then you need the "shorty" lr308 length buffer.

Typically Aero 308 uses a AR15 carbine buffer tube... and that requires a 2.5" buffer be used.

IE, the longer "308" BCG needs extra room to fully compress into it.

Just adding specific OAL buffer length.

Basic user · Oct 11, 2024

I'm using the user adjustable Odin Works 308 buffer. That's correct right?

Darryle · Oct 12, 2024

Here's some fodder for you to read, scroll down and there is a list of measurements for tubes, springs and buffers. I just know that when I built my first large frame AR, I heard the term, "There is ¢.75 difference between Large Frame AR buffer tubes", turns out if you wind up with the longer one, 3 quarters will fix the problem.

Compatibility Large Frame ARs

AMGtuned · Oct 12, 2024

Basic user said:
I'm using the user adjustable Odin Works 308 buffer. That's correct right?

Carbine RE on a large frame, requires a "308 carbine" spring and a 2.5" length buffer.

You did not get any of those things by removing them from any ar15. Maybe the tube.

The longer (3.5"?) buffer found in ar15 type rifles, will work in large frame when using an A5 RE.

The last option is a rifle length RE, 308 rifle spring and buffer.

Once you have that part figured out, then you can tackle the rest.

When i built my ar10, I used the rifle system and a prs3. I switched out the Carbine system after trying to fire some 2700fps bolt gun loads that were popping primers out in the ar10. AGB helps a lot, but is not necessary

Basic user · Oct 12, 2024

AMGtuned said:
Carbine RE on a large frame, requires a "308 carbine" spring and a 2.5" length buffer.

You did not get any of those things by removing them from any ar15. Maybe the tube.

The longer (3.5"?) buffer found in ar15 type rifles, will work in large frame when using an A5 RE.

The last option is a rifle length RE, 308 rifle spring and buffer.

Once you have that part figured out, then you can tackle the rest.

When i built my ar10, I used the rifle system and a prs3. I switched out the Carbine system after trying to fire some 2700fps bolt gun loads that were popping primers out in the ar10. AGB helps a lot, but is not necessary

That site is saying there different sizes of buffer tubes for the AR10 and 15; are they grouping the AR308 in with that?

hlee · Oct 12, 2024

Basic user said:
I took it out of my AR15 carbine. Does that make a difference? I'm thinking not.

The ar15 carbine buffer is longer than the lr308 buffer. You need a different/shorter buffer.

Darryle · Oct 12, 2024

hlee said:
The ar15 carbine buffer is longer than the lr308 buffer. You need a different/shorter buffer.

He's already too short, remove the spring and follow these instructions to eliminate the spring as the culprit

Buffer/Tube issues

AMGtuned · Oct 12, 2024

Basic user said:
That site is saying there different sizes of buffer tubes for the AR10 and 15; are they grouping the AR308 in with that?

Diameter; no, not that I'm aware of.

Length; I'm also quite certain that Carbine is carbine, a5 is a5, and rifle is rifle.

Spring and buffer are different.

Aero Precision themselves sell the carbine "kit" you need. Matter of fact, there's one on my rifle now because I didn't have another rifle system for it. It's a carbine RE, 2.5" h3 buffer, and 308 carbine spring.

msgriff · Oct 12, 2024

Wrong spring.
An M4/AR-15 carbine spring will not work with a 308 short carbine buffer (2.5" OAL) in a M4/AR-15 carbine receiver extension (7" inside depth).

The maximum allowable compressed spring length of a 308 short carbine buffer is around 2.000"-2.125".
An M4/AR-15 carbine spring with 37 coils and a wire diameter of 0.072" is 2.736" long at full compression.
An AR308 carbine spring with 26 coils and a wire diameter of 0.075" is 1.950" long at full compression.

Dimensions listed can vary between manufacturers and allowable variances/tolerances.

Basic user · Oct 13, 2024

Darryle said:
He's already too short, remove the spring and follow these instructions to eliminate the spring as the culprit

View attachment 8522618

Buffer/Tube issues

I did just that; that's how I made my guess that the current culprit is the spring. That was my guess anyhow.

mtrmn · Oct 13, 2024

Basic user said:
I took it out of my AR15 carbine. Does that make a difference? I'm thinking not.

Yes--that's the problem most likely.
I was cleaning my 308 and when I put it back together I just picked up a spring. It was for an AR15 that I also had tore down. Same problem you stated. Get a 308 carbine spring.

Shooter McGavin · Oct 16, 2024

M5 .308 Carbine Buffer Kit

Shop M5 .308 Carbine Buffer Kit, No Stock and more online at aeroprecisionusa.com. Free shipping on all orders over $99.

www.aeroprecisionusa.com

wade2big · Oct 16, 2024

Shooter McGavin said:
M5 .308 Carbine Buffer Kit

Shop M5 .308 Carbine Buffer Kit, No Stock and more online at aeroprecisionusa.com. Free shipping on all orders over $99.

www.aeroprecisionusa.com

If he uses that kit he needs to throw that light buffer away and buy this:

https://kakindustry.com/lr-308-carbine-buffer-heavy

He will be mighty unhappy if he picks up a 22” rifle gas 6.5CM upper that Aero sells and will be really disappointed if he pairs it with the Aero buffer kit quoted above while keeping the 3oz buffer.

r.tenorio671 · Oct 16, 2024

TBH, if you are using an Aero upper and lower, just purchase the Aero buffer system (tube, buffer and spring) and an Aero BCG and you should be G2G.

FWIW, the Aero .308/6.5CM nitrided bolt claims to be a "HP" bolt, i.e., smaller diameter firing pin and firing pin hole. It is smaller than the typical .308 bolts, but not as small as the JP's....

Basic user · Oct 19, 2024

Got it! The Aero Ar308 specific spring came in and it works now. So those websites that mark the recoil springs as AR15 and AR308 compatible are wrong. Now I know.

MaximusDecimusMeidius · Oct 19, 2024

this would be a good question for ben stoeger

Basic user · Nov 1, 2024

Ok, now that I had the time to fuck with this thing, the buffer's 9/16" too short; the carrier body's slamming into the lower receiver/buffer tube threads. I just dont understand lol. I can see plus or minus a quarter's worth of length, but 9/16ths inch?

r.tenorio671 · Nov 3, 2024

FWIW, others gave you valid suggested resolutions to the problems you have encountered. The easiest fix is to just use Aero buffer system parts in your Aero receiver. For this latest issue you report, it was previously suggested to use some inserted quarters in the buffer tube before inserting the buffer spring so they act as "spacers". The actual amount to use is determined by trial and error until resolved. If you are already seeing indications that the BCG has/is impacting the receiver body that the buffer tube threads into, you need to check for cracks that may have resulted. It could lead to a catastrophic failure under use.

EotS · Nov 4, 2024

Why are large framed ARs such a pita? Every one is nuanced to hell and back. Why isn't there a simple standard?

hkguns · Nov 4, 2024

They are not that difficult.

You need to pay attention to what you are doing and source your parts wisely. If you don't want to take the time to research the parts you are using, single source them to ensure compatibility and functionality.

You can't just buy a bunch of crap, from different sources, toss it together and expect it to work.

hlee · Nov 4, 2024

EotS said:
Why are large framed ARs such a pita? Every one is nuanced to hell and back. Why isn't there a simple standard?

That’s easy. Unlike the AR15, the large frame AR wasn’t adopted by the military. It’s the military adoption that created/forced the ar15 standardization.

Basic user · Nov 4, 2024

r.tenorio671 said:
FWIW, others gave you valid suggested resolutions to the problems you have encountered. The easiest fix is to just use Aero buffer system parts in your Aero receiver. For this latest issue you report, it was previously suggested to use some inserted quarters in the buffer tube before inserting the buffer spring so they act as "spacers". The actual amount to use is determined by trial and error until resolved. If you are already seeing indications that the BCG has/is impacting the receiver body that the buffer tube threads into, you need to check for cracks that may have resulted. It could lead to a catastrophic failure under use.

I do know about throwing a few quarters in to alleviate that problem, but it's more than a few quarters off; I wouldnt feel comfortable putting like 10 quarters in there to take up that space. From what I gathered I had all the correct parts, and it was just the spring that was the last part that needed correction. And I haven't fired this yet, I was just pulling the charging handle back to check function when I noticed it hit the lower and the buffer tube. It's not bad but you can see where the anno was compromised. No biggie though.

LRRPF52 · Nov 5, 2024

Basic user said:
Ok, now that I had the time to fuck with this thing, the buffer's 9/16" too short; the carrier body's slamming into the lower receiver/buffer tube threads. I just dont understand lol. I can see plus or minus a quarter's worth of length, but 9/16ths inch?

If you’re doing a carbine stock on a large frame AR, you really should be using an A5 RET (Medium), Rifle Spring, and AR-15 carbine-length heavier buffers.

RETs:

Rifle length (same on AR-15 and SR-25/LR-308/ArmaLite Inc. AR-10)
A5 RET: Midway between a rifle and carbine
Carbine RET: Standard AR-15/M4 carbine extension tube

Buffers:

AR-15 rifle RET + AR-15 Rifle buffer = GTG
AR-15/AR-10 Rifle RET + AR-10 Rifle buffer (slightly shorter than AR-15 rifle buffer) = GTG in SR-25/LR-308/new AR-10s (post 1996)
A5 RET plus AR-15 Carbine buffer = GTG in AR-10/SR-25 carbine
AR-15 RET plus tiny 2.5” LR-308 carbine buffer fits and functions somewhat in LR-308/SR-25 carbines, but not ideally.

LRRPF52 · Nov 5, 2024

Basic user said:
I do know about throwing a few quarters in to alleviate that problem, but it's more than a few quarters off; I wouldnt feel comfortable putting like 10 quarters in there to take up that space. From what I gathered I had all the correct parts, and it was just the spring that was the last part that needed correction. And I haven't fired this yet, I was just pulling the charging handle back to check function when I noticed it hit the lower and the buffer tube. It's not bad but you can see where the anno was compromised. No biggie though.

What buffer are you using? Too short of a buffer will allow the bolt carrier bottom rails to smash into the RET tower threads.

Also, BCGs designed to fit and function in one lower receiver type aren’t always compatible with others due to the rail lengths.

The whole gun really needs to be designed, engineered, tested, and refined under one roof, to one set of standards due to the significant variations in dimensions, or assembled and tested by someone with the experience necessary to make it work.

Just within the old DPMS brand and LR-308 series, there were 3 different receiver heights, for example.

SR-25s have undergone many changes and improvements over the years since they were first made in 1989.

ArmaLite Inc. (re-branded Eagle Arms) have gone through several iterations as well, with two main receiver types using different magazines.

Aero made the receiver sets for ArmaLite Inc. They mainly sourced BCGs, extensions, barrels, and small parts from other companies for their M5 series, which have also undergone several changes since their introduction in the race to the bottom of price points and performance.

Basic user · Nov 5, 2024

LRRPF52 said:
What buffer are you using? Too short of a buffer will allow the bolt carrier bottom rails to smash into the RET tower threads.

Also, BCGs designed to fit and function in one lower receiver type aren’t always compatible with others due to the rail lengths.

The whole gun really needs to be designed, engineered, tested, and refined under one roof, to one set of standards due to the significant variations in dimensions, or assembled and tested by someone with the experience necessary to make it work.

Just within the old DPMS brand and LR-308 series, there were 3 different receiver heights, for example.

SR-25s have undergone many changes and improvements over the years since they were first made in 1989.

ArmaLite Inc. (re-branded Eagle Arms) have gone through several iterations as well, with two main receiver types using different magazines.

Aero made the receiver sets for ArmaLite Inc. They mainly sourced BCGs, extensions, barrels, and small parts from other companies for their M5 series, which have also undergone several changes since their introduction in the race to the bottom of price points and performance.

I sourced a standard issue strike industries ar15 carbine buffer tube, aero's ar308 buffer spring, and odin works ar308 adjustable buffer.

r.tenorio671 · Nov 5, 2024

...here are some pics of the Aero parts from 2015. My M5 build has worked flawlessly since day one using M118, FGGM 168, various handloads in 155-178gn loads down a 16" CLGS tube w/SLR AGB.

There are also some potential "reliable functioning" issues related to choice of bolt catch.
YMMV

r.tenorio671 · Nov 5, 2024

Bolt catch info from 2015...

Basic user · Nov 5, 2024

r.tenorio671 said:
...here are some pics of the Aero parts from 2015. My M5 build has worked flawlessly since day one using M118, FGGM 168, various handloads in 155-178gn loads down a 16" CLGS tube w/SLR AGB.

There are also some potential "reliable functioning" issues related to choice of bolt catch.
YMMV

On that buffer; is that beginning on the 13" mark?

Basic user · Nov 9, 2024

Here's what I can't figure out. (on 556 ar15's anyways, the back of the carrier tail hit's up against the buffer, and the buffers rubber top hits up against the back of the buffer tube, to stop rearward travel; the spring is not supposed to be part of that equation, and it isn't; it can't be the spring)

On Aero Precisions website, they say their AR308 carbine buffer set puts a 2.5" buffer in there. How, when mine's already 2.5 inches long, and I'm already using their spring, which does fit right where it's supposed to fit, which is fully compressed but NOT solid, and resting in between the buffer's 'head' if you will, and a 'mil spec' buffer tube bottom which I'm assuming is the standard issue 556 AR15 carbine buffer tube.

So my question is, which I cant' see it being true but I have to ask; does the AR308 not rely on the carrier tail and the buffer being kind of one piece, and the travel limiter is the buffer smacking the back of the tube? And instead rely on a really strong spring to keep the carrier from contacting the lower? Because I have all the same components as Aero Precision suggests and it still doesn't make sense.

LRRPF52 · Nov 9, 2024

Basic user said:
Here's what I can't figure out. (on 556 ar15's anyways, the back of the carrier tail hit's up against the buffer, and the buffers rubber top hits up against the back of the buffer tube, to stop rearward travel; the spring is not supposed to be part of that equation, and it isn't; it can't be the spring)

On Aero Precisions website, they say their AR308 carbine buffer set puts a 2.5" buffer in there. How, when mine's already 2.5 inches long, and I'm already using their spring, which does fit right where it's supposed to fit, which is fully compressed but NOT solid, and resting in between the buffer's 'head' if you will, and a 'mil spec' buffer tube bottom which I'm assuming is the standard issue 556 AR15 carbine buffer tube.

So my question is, which I cant' see it being true but I have to ask; does the AR308 not rely on the carrier tail and the buffer being kind of one piece, and the travel limiter is the buffer smacking the back of the tube? And instead rely on a really strong spring to keep the carrier from contacting the lower? Because I have all the same components as Aero Precision suggests and it still doesn't make sense.

In mechanical engineering, one of the no-go principles of springs is that the coils should never impact each other during the cycle of the system, whatever it may be. Coil impact leads to early spring wear and failure.

The buffer bumper is not only the limiter in the AR-15 and AR-10, but contributes to forward momentum in the return stroke when it bounces off the back inside of the RET. It doesn’t have to do that if spring weight prevents it from doing so, but that’s what it does with full gassing and complete travel.

It’s why there is a specification set in the TDP on the slightly-flexible elastomer for the buffer bumper.

But I would not use the 2.5” buffer in the LR-308 with a standard carbine RET, because it limits your spring length/weight.

This is one of the reasons you want the A5 mid-length RET, which allows you to use a rifle-length spring and heavier AR-15 carbine buffer. Buffer weight is meant to counter carrier-bounce in full auto, so isn’t as important as people think in attempts at cyclic rate reduction.

The action spring is more of a factor in all of this, and rifle-length, heavy weight springs are what the large frame BCG really needs for reliable function.

mtrmn · Nov 9, 2024

Remove the buffer and spring. Use a ruler or tape to measure the inside depth of the buffer tube.

A std AR15 carbine tube should be very close to 7" deep inside. (6-15/16 is the Armalite spec) If this is what you have on a 308 carbine, then the buffer has to be 2 1/2" long. The spring must be for a 308 carbine, NOT an AR15 spring. The AR15 spring will prevent the BCG from traveling far enough back to get past the bolt catch.

IMO-the better option on buffer tubes will have a 7-5/8" internal depth. This tube will use a std length AR15 carbine buffer which is 3-1/4" long. H3 weight with a rifle-length 308 spring. This will get you there all in 1 kit:

Armalite AR10 7 POS REC Ext kit assy, No Stock AR10REKIT01 ArmaLite Parts | EveryGunPart.com

AR10 7 POS REC Ext kit assy, No Stock Mil Spec MPN: AR10REKIT01 UPC: 651984020142 ArmaLite AR10 7 Pos Receiver Extention Kit, Ambi Plate, No Stock AR10REKIT01 All the parts need to convert your lower Armalite AR10 Lower receiver t

everygunpart.com

Ronin22 · Nov 9, 2024

Basic user said:
Decided on a collapsible (carbine) stock for this new Aero 6.5 gas gun build and the bolt won't go all the way back to lock open; it's about a quarter inch short. The buffer springs bottoming out and I can not figure out why. I know the whole "there's no single spec for ar308s/ar10s" thing, but finding out what's compatible and what's not seems to be pretty challenging. I keep seeing that the buffer tubes and buffer springs are compatible, but buffers are not. So, then how does this carrier tail bottom out? The tube's Strike Industries, the buffer's an adjustable Expo Arms, and the springs a no name AR15.
So during my research, the only thing I found was that on Aero's website, their 308 carbine spring has a different part number than their AR15 carbine spring... but when shopping on all the ar parts websites, they all list (except for aero) all the same springs for the 308s and 556s...
Called Brownells and they list the OAL's for the springs in their notes, and the 308 springs thicker and longer lol. Would Im imagining would exacerbate my issue (if those measurements are correct) or does the 308 spring have so fewer coils that it maintains a shorter compressed OAL...

Has anyone built one from the ground up on an Aero set and run into this same problem?

Aero carbine receiver extensions/buffer tubes are 7". You use a buffer longer than they sell and you got your problems.

r.tenorio671 · Nov 10, 2024

Basic user said:
On that buffer; is that beginning on the 13" mark?

Yes, the Aero buffer pictured is 2.50" in length.

longknives · Nov 11, 2024

mtrmn nailed it!

Basic user · Sunday at 9:19 PM

Figured it out. The strike industries buffer tubes are about .135" deeper than all the other buffer tubes. I guess my initial depth test must have missed this.

longknives · Monday at 4:42 PM

Try putting 2 quarters in the buffer tube then add the spring and buffer.

Basic user · Monday at 6:16 PM

I think it took three quarters actually, and it was more than I was willing to put up with. I know from building a lot of standard ar15's that sometimes you may need a quarter, or even the little weird ass shim jp ships with every package he sells, but three is just out of spec plain and simple. No way I'm doing that or doing it with a friends build either lol. I'd look like a weirdo if he pulls his shit out to clean and out flys fucking 75 cents lol.

eastexsteve · Monday at 7:17 PM

mtrmn said:
IMO-the better option on buffer tubes will have a 7-5/8" internal depth. This tube will use a std length AR15 carbine buffer which is 3-1/4" long. H3 weight with a rifle-length 308 spring.

FWIW, my small frame AR10 has the above components. Except, the H3 buffer is on the heavy side at 5.7 ounces. However, it throws brass exactly where it should.

joe777 · Monday at 8:27 PM

Basic user said:
Figured it out. The strike industries buffer tubes are about .135" deeper than all the other buffer tubes. I guess my initial depth test must have missed this.

Wouldn't that allow the bolt to still lock back per your original post?

Basic user · Wednesday at 1:32 PM

It would, but my original problem back then was that I didn't have enough travel due to me putting in a standard 556 AR15 carbine buffer due to some bad intel. Once I fit an AR308 specific buffer in there I then was faced with another problem of the opposite effect. It was the buffer tube.

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Does Aero Precision AR308s do anything different than the others?

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