Buffers, receiver extensions, stocks, and springs

[Spart]

Caveats

RRA LAR-8 and Bushmaster BAR-10 308 (pre-2010) caveat: these guns have a proprietary design with proprietary buffers. RRA goes out of their way to state this on their website. The design started life as the Bushmaster BAR-10 and the design was sold to RRA, who have continued marketing it as the LAR-8 it after making small changes. Bushmaster has been selling rebadged DPMS LR-308's as the Bushmaster ORC and Hunter 308 lines since 2010, and this caveat does not apply to those rifles. Bushmaster BAR-10 and RRA LAR-8 actions have a longer stroke, and thus a shorter buffer compared to every other 308 AR out there. Take a look at this picture of an LAR-8, the receivers are a bit odd (long) in proportion to most 308 AR's. The reason is the proprietary bolt catch/release, which is positioned similarly to an ACR or Beretta ARX. Therefore nothing in this section is applicable to these rifles, however Slash does sell buffers for these rifles on his website.

HK MR762/HK417 caveat: The receiver extension on these rifles is much larger in diameter and completely different and incompatible with all aftermarket stocks, springs, and buffers because you suck and HK hates you.

Action length 308 vs. 5.56

The first thing you need to know is that AR-10 and 308 AR rifles have a longer action length than AR-15 rifles. The actual action length isn't super important (and will vary even between rifles of the same make and model due to variations in the way the receiver and receiver extension are threaded) but it is important to know that it is around 3/4" longer than an AR-15 action. This means that the BCG is moving an additional 3/4" into the stock, the charging handle is moving an additional 3/4" to the rear, and for these reasons you cannot swap an AR-15 stock, buffer and action spring system wholesale to a 308 rifle. Many AR-15 parts can be used with 308 rifles when combined with the correct 308 specific components.

Rifle stocks

In general, rifle stocks and receiver extensions for AR-10/308AR's are identical to AR-15 rifle stocks. Due to the longer action length, a stock with a cheek riser that works on an AR-15 may interfere with a 308 AR's charging handle. Magpul sells a version of their PRS for 308 AR's specifically to address this issue, however the standard AR-15 PRS can be modified by cutting the cheek riser, and I assume other stocks with adjustable comb height could be similarly modified.

Rifle buffers for 308 rifle stocks are not the same as AR-15 components, they are 308-specific. Standard(ish) 308 rifle buffers are slightly heavier and about 3/4" shorter than the AR-15 rifle buffer to accommodate the longer action stroke.

ArmaLite EA1090 - Listed as 5.1875", 5.4 ounces

DMPS 308-BS-11 - Listed as 5.25", 5.4112 ounces

Slash Heavy Buffer - Listed as 5.200", 10.0 ounces.

Action springs for 308 rifle stocks are also not the same as AR-15 components. They are higher rate springs. Slash recommends the ArmaLite part FWIW.

ArmaLite EA1095

DPMS 308-BS-10B

TUBB ARBUFSR

JP JPS-OSR.308

JP JPSCS-10 - proprietary captured system with springs, must use included spacer, alternative springs available as JPSCS-PACK10

JP JPSCS-10K - as above with alternative springs, must use included spacer.

Carbine stocks

Here's where things get muddy. There are two competing standards for carbine stock systems in 308 AR rifles, and it goes beyond the commercial/mil-spec dichotomy found in AR-15 carbine stock systems.

The first system, which I will refer to as the ArmaLite AR-10 style carbine stock system or "ArmaLite style system" for short, uses AR-15 carbine buffers. As stated earlier, the stroke of a 308 AR's action (and thus the stroke of the BCG inside the receiver extension) is around 3/4" longer than in an AR-15, and so for this system there are 308-specific receiver extensions that are around 7 3/4" long internally vs. 7" internally on the AR-15 variety.

The second system, which I will refer to as the DPMS LR-308 style carbine stock system or "DMPS style system" for short, uses AR-15 carbine receiver extensions. Again, due to the longer stroke of the 308 AR action, a 308-specific buffer is used that is around 3/4" shorter than an AR-15 carbine buffer.

This picture illustrates the difference between ArmaLite AR-10 style carbine receiver extensions and the AR-15 carbine receiver extension used in the DPMS style system:

This picture illustrates the difference between DPMS LR-308 style carbine buffers and the AR-15 carbine buffer used in the ArmaLite style system:

ArmaLite AR-10 style carbine stock system

ArmaLite and several other manufacturers (complete list at the bottom of this section) utilize an AR-15 carbine buffer in conjunction with a 308-specific action spring and receiver extension. You can use any carbine stock that matches the OD of your tube (commercial vs. mil-spec) however see the notes below on which setups will either not fully collapse or over collapse.

The buffer ArmaLite uses is identical to an AR-15 H3 buffer. It contains three tungsten weights and weighs 5.4 ounces. Other manufacturers that use this system style may use differently weighted buffers from standard carbine all the way through H3. If you need to go heavier than an H3, the AR-15 buffers listed on Slash's site will work and he recommends the XH.

Action springs are 308-specific and are of a higher rate than AR-15 carbine springs.

ArmaLite EA1095 - This is the same as for the ArmaLite rifle stock and is what I use in my carbine stock.

TUBB ARBUFSR

Sprinco "RED" Spring

Receiver extensions are 308-specific and are about 3/4" longer than AR-15 receiver extensions (see pic above.) Depending on the receiver extension used, you can fully collapse elongated stocks such as the VLTOR Emod and MagPul ACS/ACS-L/STR stocks. On the flip side, if you use a stock with no back (visible hole in the rear of the stock as with a mil-spec collapsible stock) it is possible to over collapse a standard length stock on an extension that allows an elongated stock to fully collapse.

Several manufacturers offer the 308-specific receiver extension for sale separately:

VLTOR RE-A5 - mil-spec OD, allows elongated stocks to fully collapse.

VLTOR RE-10/A5SR - mil-spec OD, will not allow elongated stocks to fully collapse or standard stocks to over collapse.

RE-PA5: VLTOR A5 Pistol TubeArmaLite 10207025 - mil-spec OD, will not allow elongated stocks to fully collapse or standard stocks to over collapse.

POF 00517 Buffer Tube, Anti-Tilt (7-Position) - mil-spec OD, allows elongated stocks to fully collapse. Anti-tilt feature for op-rod guns.

LaRue LT629-15C for 7.62 - Select the 7.62 option, mil-spec OD, will not allow elongated stocks to fully collapse or a standard stock to over collapse.

PWS Enhanced Buffer Tube - .308 - mil-spec OD, will not allow elongated stocks to fully collapse or a standard stock to over collapse.

Incidentally, the non-VLTOR tubes will work with VLTOR A5 buffers in an AR-15, despite what Vltor's website says about this. Could be of interest to folks who want an anti-tilt receiver extension with an A5 buffer in an op-rod AR-15 - POF has you covered.

Stocks are standard AR-15, and all of the receiver extensions I have found utilize the mil-spec diameter, so you'd have to use a mil-spec stock. As mentioned above, if you choose the right receiver extension you can fully collapse an elongated stock such as the VLTOR EMOD and MagPul ACS/ACS-L/STR. If you wish to use a stock with an open rear, make sure to choose a receiver extension that will not let you over-collapse the stock.

Complete stock kits are available from additional manufacturers that do not offer the parts individually:

KAC offers the "M110 Collapsible Stock Kit," part number 25951. It's not listed on their website but you can purchase it through their dealer network, e.g. here. This kit appears unique in that it utilizes a ten position receiver extension, whereas the other receiver extensions available are either seven or six position. It appears to allow elongated stocks to fully collapse and comes with the MagPul ACS.

LMT offers a few different kits:

LMT L7LA2CT - I can only assume this is mil-spec OD since LMT's SOPMOD stock is mil-spec, the stock is not long enough to fully collapse.

LMT L7LA2C - As above only in black.

LMT DMR308-KIT - same guts as above but with LMT's DMR stock that resembles a MagPul PRS. It slips on over the carbine style receiver extension whereas the PRS utilizes a rifle receiver extension.

PSA offers a couple of kits:

PSA PA10 .308 Classic Stock Kit Black - mil-spec tube, AR-15 H buffer, unknown spring and M4-style stock. Tube will not allow stock to over collapse.

PSA PA10 .308 MOE Stock Kit Black - as above with MOE stock.

The following rifle lines utilize the ArmaLite style system, albeit with their own flavor of buffer weights and springs:

ArmaLite AR-10

KAC M110 (when equipped with above-referenced kit)

KAC 16” SR25 EC and ECC, see here.

LMT LM308

POF mfg Oct 2012 or later, see note at bottom of page here.

S&W M&P10 - known to use smaller gas ports and lighter buffers. Rifle stock systems use the carbine buffer/spring and a spacer.

LaRue 7.62

PSA PA-10

Sig 716 - certain models use the DPMS style system instead.

Ruger SR-762

PWS MK2

Noveske N6

DPMS LR-308 style carbine stock system

DPMS and several other manufactures (complete list at bottom of this section) utilize an AR-15 receiver extension in conjunction with a 308-specific buffer and action spring. You can use any carbine stock that matches the OD of your tube (commercial vs. mil-spec.)

The buffer DPMS uses looks like a cut-down AR-15 carbine buffer. It is around 3/4" shorter. You can see a comparison pic above.

There are a few options for buffers:

DPMS 308-CS-10B - this is the standard DPMS buffer.

Slash CAR-10 buffers - Available in 5.5oz and XH 6.5oz. Slash recommends his proprietary spring for use with these.

Buffer Technologies KynSHOT KSRB5004 - Hydraulic recoil damper.

JP JPSCS-10 - proprietary captured system with springs, alternative springs available as JPSCS-PACK10

JP JPSCS-10K - as above with alternative springs.

Other companies may make something very similar to the DPMS buffer, but they've gotten pretty generic by now similar to mil-spec AR-15 buffers.

Action springs are specific to the DPMS LR-308 style system. Springs that work in the ArmaLite AR-10 style system will not work with the DPMS style system.

DPMS 308-CS-10A

JP JPS-OSC.308

JP JPSCS-10 - proprietary captured system with buffer weights, alternative springs available as JPSCS-PACK10

JP JPSCS-10K - as above with alternative springs.

Slash .308 CARBINE Buffer Spring - Custom

Sprinco "ORANGE" Spring

Any AR-15 carbine receiver extension will work with the above components, commercial or mil-spec. If you want to run a MagPul UBR on your rifle, the DPMS style system is the only way to go.

The following rifle lines utilize the DPMS style system, albeit with their own flavor of buffer weights and springs:

DPMS LR-308 Gen 1/GII Rifles

Bushmaster 2010+

Remington

POF prior to Oct 2012

Black Rain Ordinance

CMMG

Colt 901

Fulton Armory FAR-308

LWRC REPR

Wilson Combat .308

JP LRP-07

SIG 716 - certain models use the ArmaLite style system instead.

Rainier Arms RA308

Aero Precision M5E1

KAC SR-25k

KAC SR-25BR (battle rifle, preceded EM Carbine)

Seekins Precision SP-10

DRD M762

DRD G762

[washguy]

Spart    no snarky comment from me...well done  <lmao>  Wash

[Spart]

Cross posting from AR15.com, just put too much time into compiling the info to let it die.

 

Any additional info/updates/corrections post em up, and I'll edit the OP.

[392heminut]

Great post! <thumbsup>

[BLKSHEEP]

Great post! <thumbsup>

+1 This should be verified & pinned.

[FaRKle!]

Not bad, but I spy some errors...

Caveats

 DPMS GII LR-308 caveat: these guns have a new proprietary design, and according to DPMS their carbine buffer and spring do not interchange with the gen one rifles.  I assume that none of this information pertains to the GII rifles.  It could be that they have gone to a system that splits the difference between ArmaLite and DPMS in order to achieve a system that is 100% proprietary in nature, but that's just conjecture on my part.

GII rifles use an LR308 buffer system. The parts are interchangeable.

See bottom two lines of the following chart:

...you cannot swap an AR-15 stock, buffer and action spring system wholesale to a 308 rifle.

In general you CAN use an AR-15 stock on a .308 AR. It may not lock up on the most closed position however depending on the receiver extension used.

 

Incidentally, the non-VLTOR tubes will probably work with VLTOR A5 buffers in an AR-15.  I have not verified this, but I don't know why it wouldn't work.  Could be of interest to folks who want an anti-tilt receiver extension with an A5 buffer in an op-rod AR-15 - POF may have you covered.

I use others with the A5 and have verified this (LMT MWS receiver extension, POF 7-position receiver extension, and PWS EBT .308 receiver extension).

 

The following rifle lines utilize the ArmaLite style system, albeit with their own flavor of buffer weights and springs:

 

PSA PA-10

Some PA-10 kits came with LR-308 pattern buffer systems.

Action springs are specific to the DPMS LR-308 style system.  Springs that work in the ArmaLite AR-10 style system will not work with the DPMS style system.

Since it wasn't specified earlier, with a rifle/A1/A2 receiver extension the rifle buffer and action spring are identical between the AR-10 and LR308.

The following rifle lines utilize the DPMS style system, albeit with their own flavor of buffer weights and springs:

DPMS LR308 GII included.

[98Z5V]

^^^ What he said.  Add that info in, and you have a pretty comprehensive post.

[Spart]

Not bad, but I spy some errors...

GII rifles use an LR308 buffer system. The parts are interchangeable.

See bottom two lines of the following chart:

Nice! The DPMS rep on AR15.com clearly has no clue, but that's nothing new...  Updated.

 

In general you CAN use an AR-15 stock on a .308 AR. It may not lock up on the most closed position however depending on the receiver extension used.

Read the whole sentence you quoted again. What I said was correct: you cannot swap an AR-15 stock, buffer and action spring system wholesale to a 308 rifle.

 

 

I use others with the A5 and have verified this (LMT MWS receiver extension, POF 7-position receiver extension, and PWS EBT .308 receiver extension).

Good to know, I updated that section.

 

Some PA-10 kits came with LR-308 pattern buffer systems.

By mistake, apparently. Read through this thread, they put the DPMS style short buffer in the ArmaLite style long tube.  That means the shoulder of the BCG crashes into the ring on the lower receiver. Oops!  PSA is replacing all of those.  Apparently somebody at PSA thought you could build a 308 AR by just ordering parts that were labeled "308" and slapping them together, not testing, and shipping to customers.  You get what you pay for...

 

Since it wasn't specified earlier, with a rifle/A1/A2 receiver extension the rifle buffer and action spring are identical between the AR-10 and LR308.

 

I don't know about identical, according to the manufacturer's spec (DPMS/ArmaLite) the length is slightly different. I did include that info under the "Rifle Stocks" section.

I would say the rifle buffers could be used interchangeably since the small difference in length (if it isn't a misprint by one mfg or the other) probably doesn't matter, and they're probably the same weight. The springs are definitely different, I have both the ArmaLite and DPMS rifle springs in my possession. They are not the same. I use the ArmaLite version.

 

DPMS LR308 GII included.

Updated!

[FaRKle!]

I don't know about identical, according to the manufacturer's spec (DPMS/ArmaLite) the length is slightly different. I did include that info under the "Rifle Stocks" section.

I would say the rifle buffers could be used interchangeably since the small difference in length (if it isn't a misprint by one mfg or the other) probably doesn't matter, and they're probably the same weight. The springs are definitely different, I have both the ArmaLite and DPMS rifle springs in my possession. They are not the same. I use the ArmaLite version.

If the rifle receiver extensions are interchangeable, and the buffers are interchangeable, then the action springs are also interchangeable.

[Spart]

Just because it will fit doesn't mean it's a good idea. The spring rates may be different between manufacturers. This would correlate to how the gas system is setup for a reliable rifle. The lengths are certainly different. The ArmaLite spring is engineered to work with their carbine system for one, whereas DPMS has separate carbine and rifle springs. I believe the DPMS spring is longer, I can verify that when I'm at home and even take some pics if necessary.

I run a Vltor A5 tube, generic H3 AR-15 carbine buffer, and ArmaLite EA1095 spring in my rifle. It's a bit of a moot point for me on the buffer weight and spring rate, since I have an adjustable gas block. This is the first setup that I tried and it's functioned 100%, so I see no need to change it around. It would be interesting to see if the DPMS rifle spring that I have would function fine in an ArmaLite style carbine system, though.

[FaRKle!]

Just because it will fit doesn't mean it's a good idea. The spring rates may be different between manufacturers. This would correlate to how the gas system is setup for a reliable rifle. The lengths are certainly different. The ArmaLite spring is engineered to work with their carbine system for one, whereas DPMS has separate carbine and rifle springs. I believe the DPMS spring is longer, I can verify that when I'm at home and even take some pics if necessary.

I run a Vltor A5 tube, generic H3 AR-15 carbine buffer, and ArmaLite EA1095 spring in my rifle. It's a bit of a moot point for me on the buffer weight and spring rate, since I have an adjustable gas block. This is the first setup that I tried and it's functioned 100%, so I see no need to change it around. It would be interesting to see if the DPMS rifle spring that I have would function fine in an ArmaLite style carbine system, though.

I think you're missing the whole context. On the surface and only narrowly focusing on "does it fit" yes, there is some caution that should be used, that said if you look at both of the rifle gas systems there's just a slight difference in the gas port location (<0.5") that doesn't amount to much since the pressure change per distance is much less once you get that far out (not like going from a carbine length to a mid-length location). Since the buffers weigh the same and the receiver extension is the same length, the springs must be very similar. Just because one spring is a bit longer or has more coils doesn't necessarily mean it has a wildly different spring rate. You'd probably have more variation when swapping around springs on an AR-15 (flat wire, to chrome-silicon, to extra power...) There's plenty of margin.

You'll notice that one of your references (Slash) recommends using the AR-10 spring and rifle buffer for LR308 rifles since they're interchangeable.

The AR-10 action spring wasn't engineered to work with the AR-10 carbine system, rather the AR-10 carbine system was engineered to work with the AR-10 action spring. The AR-10 system naturally achieves what the VLTOR A5 system does for the AR-15. Maintaining the rifle spring in a carbine format. The LR308 carbine spring is necessarily different from the rifle spring due to the shorter travel of the AR-15 carbine receiver extension.

Edited December 2, 2014 by FaRKle!

[Spart]

In any event, my original post doesn't say one way or the other on interchangeability between brands for the springs. The springs are bound to be different in small ways, and the differences can be a good thing if you are looking to tune your rifle without the aid of adjustable gas. I wish I knew which springs were heavier, how the cyclic rate would be affected, and so on. But I don't, and I also won't make the claim that two different manufacturers of a spring for a platform that has no standard are "identical" as you assert in your first post.

[Spart]

I think you're missing the whole context. On the surface and only narrowly focusing on "does it fit" yes, there is some caution that should be used, that said if you look at both of the rifle gas systems there's just a slight difference in the gas port location (<0.5") that doesn't amount to much since the pressure change per distance is much less once you get that far out (not like going from a carbine length to a mid-length location).

I would agree that the slight difference in port location is just that, but I have yet to read anything comparing gas port sizes, which are crucial and can vary quite a bit between manufacturers.

 

Since the buffers weigh the same and the receiver extension is the same length, the springs must be very similar. Just because one spring is a bit longer or has more coils doesn't necessarily mean it has a wildly different spring rate.

Why make an assumption and put it out there as fact?

 

You'll notice that one of your references (Slash) recommends using the AR-10 spring and rifle buffer for LR308 rifles since they're interchangeable.

Probably because DPMS parts can tend to suck? I also tend to avoid DPMS parts. My experience with the ArmaLite EA1095 spring is good and the spare I purchased for my rifle build is also an EA1095. Sounds like Slash has good advice.

 

The AR-10 action spring wasn't engineered to work with the AR-10 carbine system, rather the AR-10 carbine system was engineered to work with the AR-10 action spring.

How do you know that for sure (chicken/egg argument) and what difference does it make? Are you sure ArmaLite didn't specify a new spring for the rifle when they started doing carbines? Maybe DPMS patterned their spring after the original ArmaLite spring and ArmaLite changed theirs...

Regardless the springs are different, no matter how we got here. I really don't want to fill my post full of conjecture.

 

The AR-10 system naturally achieves what the VLTOR A5 system does for the AR-15. Maintaining the rifle spring in a carbine format. The LR308 carbine spring is necessarily different from the rifle spring due to the shorter travel of the AR-15 carbine receiver extension.

This is exactly why I chose to go with the ArmaLite style system. I am also a fan of the A5 system, all of my AR-15's have one.

While I have no data to back it up, I'm very suspicious of the DPMS design and I suspect the ArmaLite design is the superior of the two.

Edited December 2, 2014 by Spart

[Spart]

Added PWS stuff to the OP. Good god, that is an expensive receiver extension.

[edgecrusher]

Added PWS stuff to the OP. Good god, that is an expensive receiver extension.

I like using them, it sure makes it easy

[Spart]

For $200 I can be bothered to bust out the castle nut wrench. That's crazy talk.

[FaRKle!]

For $200 I can be bothered to bust out the castle nut wrench. That's crazy talk.

Edge's got a factory rifle, as do I ;)

[survivalshop]

 Good info .

[Spart]

Edge's got a factory rifle, as do I ;)

Their factory rifles honestly aren't bad value. But just knowing I got a $200 receiver extension and a $60 trigger vs. a $45 receiver extension and a $215 trigger would always nag me.

Can you confirm that the receiver extension is one of the variety that doesn't let open-back stocks over collapse, nor extended stocks fully collapse? The response I got back from PWS was "our customers haven't complained." Real helpful guys, way to not answer my question.

Edited December 3, 2014 by Spart

[FaRKle!]

Their factory rifles honestly aren't bad value. But just knowing I got a $200 receiver extension and a $60 trigger vs. a $45 receiver extension and a $215 trigger would always nag me.

It's like KAC including the vanilla milspec grip on their rifles. They know you're going to change it out anyways to your preferred flavor so they just stick a cheap, but decent one in there (ALG QMS, which I happily put into another gun).

Can you confirm that the receiver extension is one of the variety that doesn't let open-back stocks over collapse, nor extended stocks fully collapse? The response I got back from PWS was "our customers haven't complained." Real helpful guys, way to not answer my question.

Yes, the stock travel length is the standard 6 position as with an AR-15 carbine receiver extension. I replaced it with a VLTOR A5 7-position and later a POF 7-position anti-tilt to get a shorter LOP.

[Spart]

The QMS is great for the parts bin, indeed. I have one and it's superb for what it is, but I'm just spoiled on Geissele two stage triggers. They completely ruined me.

Thanks for the info, I'll update the OP.

[Spart]

Can anyone think of any other rifle lines that need added to either carbine stock category?

[FaRKle!]

Can anyone think of any other rifle lines that need added to either carbine stock category?

AR-10:

-Noveske N6

LR-308:

-Rainier Arms RA308

-Aero Precision M5E1

-KAC SR-25k

-KAC SR-25BR (battle rifle, preceded EM Carbine)

-Seekins Precision SP-10

-DRD M762

-DRD G762

Edited December 4, 2014 by FaRKle!

[Spart]

Updated.

[Spart]

VLTOR now has a pistol-style A5 extension, but I can't figure out how to edit the OP.  Is editing disabled after a certain amount of time or am I just lost?
 

RE-PA5: VLTOR A5 Pistol Tube