First of all let me say that I have read this post by ole-cowboy and I think there is some confusion! Namely, it's mentioned that an RRA stock set was used. Well, RRA 308 AR's are kind of their own thing. RRA goes out of their way to state this on their website, and Slash of HeavyBuffers.com mentions that RRA LAR-8's aren't compatible with his AR-10/308 AR heavy buffers.

I got to looking into this, and a quick read through this thread confirmed my suspicion: RRA LAR-8 actions have a longer stroke and charging handle, and thus a shorter buffer compared to every other 308 AR out there. Just look at a 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. Bingo! Part of ole-cowboy's problem is RRA's proprietary system that's compatible with nothing else!

So I got to thinking, and I'm pretty sure I figured out where we're all wrong. S&W's answer to everything seems to be "it's proprietary" but as they say, great artists steal.

This much is for sure:

• S&W uses a standard 3.25" AR-15 carbine buffer much like the ArmaLite-style setup, but instead of an H3 they appear to use a standard weight buffer based on info in this thread.
• M&P10s with a collapsible stock have an elongated receiver extension compared to an AR-15, similar to the Vltor A5 or ArmaLite AR-10 carbine receiver extension, but the exact length is unknown. This is in contrast to the DPMS 308 style collapsible stock system, where an AR-15 receiver extension is used in combination with a shorter than normal (2.5" vs 3.25") carbine buffer. The M&P10 receiver extension is a six position with a mil-spec OD, and does not allow longer stocks like the EMOD/ACS/STR to collapse fully (which the Vltor A5 does.)
• They (S&W) do not recommend using a rifle-length receiver extension and AR-10/AR-308 rifle buffer.
• In lieu of an AR-10/AR-308 rifle buffer, they have an optional spacer which - when combined with the 3.25" buffer from the collapsible stock - allows end users to install rifle-length stocks and receiver extensions. somebody measured this spacer at 1.990"

Every enduser report I've read seems to have bought the S&W advice that you can't use an AR-10/AR-308 rifle buffer.

However, based on the 1.990" figure for the S&W spacer, we can do some math and figure out whether an AR-10/AR-308 rifle buffer in a rifle-length receiver extension would work dimensionally. Unfortunately, the length of an AR-10/AR-308 rifle buffer doesn't seem to be precisely agreed upon.

• The DPMS version is listed at 5.285" on their website and on vendor websites. This makes it 2.035" longer than the 3.25" carbine buffer, meaning it would allow for a 0.045" shorter stroke than the carbine buffer and S&W spacer.
• The ArmaLite AR-10 rifle buffer is listed at 5.1875 on their website. This makes it 1.9375" longer than the 3.25" carbine buffer, meaning it would allow for a 0.0525" longer stroke than the carbine buffer and S&W spacer.
• Slash (HeavyBuffers.com) splits the difference at 5.200" for his AR-10/AR-308 rifle buffers (compatible with both DPMS and ArmaLite) according to his chart. This makes it 1.95" longer than the 3.25" carbine buffer, meaning it would allow for a 0.04" longer stroke than the carbine buffer and S&W spacer

If all the measurements are correct, a 5.24" AR-10/AR-308 rifle buffer would produce the same stroke as a 3.25" carbine buffer and 1.99" S&W spacer. However, no such beast appears to exist.

Since we are talking about at most just under 1/16" difference here, I'm going to go out on a limb and say that all of the rifle buffers may work (except RRA, obviously!) If I was truly concerned, I would use an ArmaLite or Slash buffer since a little less stroke might cause failures to lock back on an empty mag, a little more stroke probably wouldn't hurt anything.

The weighting could be a problem, since if S&W is getting away with a standard carbine buffer rather than an H3 like other manufacturers use, a rifle buffer might be too heavy. Then again, the M&P10 might be under buffered, so the heavier rifle buffer could be an improvement?

Now, this begs the question, going back to this point:

• They (S&W) do not recommend using a rifle-length receiver extension and AR-10/AR-308 rifle buffer.

Could it be that this is solely because they don't make a 308 AR rifle buffer, and have a policy against recommending parts designed for other 308 AR rifles?

Hmm.... Who wants to test this? ArmaLite rifle buffers aren't too spendy...

Spart, in your analysis of buffers, you may have overlooked the position of the spring seats on the buffers and how this affects the spring's "free length" and its overall dynamics.

As it stands, if you use S&W's spacer with the MP10 carbine spring/buffer in the A2 extension tube, you maintain the same spring rate/power in accordance with the OEM design of the rifle.

That being said, I currently run my MP10 with a DPMS A2 extension tube, the original MP10 spring, an Enidine 308 rifle buffer and a SLR DA7 adjustable gas block. Although the length of the rifle buffer is relatively the same as the length of the carbine buffer and spacer combined, the offset of the spring seat on the rifle buffer yields a longer "free length" on the spring. Albeit minor, the spring's dynamics are changed as a result, which could affect the cycling and reliability of the rifle to consistently engage battery under varying operating conditions. Fortunately, this configuration, with the gas block properly adjusted to cycle it, has been cycled flawlessly through 200 rounds of CBC 168 grain BTHP ammo ... all fired in 2 sessions thus far in 30 degree weather, without any cleaning or boresnaking and just the first application of Frog Lube.

Agreed, however there are standard rifle springs designed for this purpose, as well as reduced and extra power springs.

Something I failed to mention, ArmaLite (and possibly other brands with the same setup) recommends the use of the same spring for both AR10 rifle and standard length AR15 carbine buffers. DPMS uses a special spring for their shorter AR10 carbine buffer, but S&W's setup is similar to ArmaLite's, not DPMS.

looks like you did a lot more fiddling with a lot more parts than I was able to do spart.

With using my RRA stock, I didn't take the S&W solution verbatim, just the reason behind it.

I stress many times over that I am not a smith,
my understanding is that the buffer "bumper" is to make contact with the bottom of the tube and travel rearward far enough to travel past the bolt hold open
In the post where the lightbulb went off for me "stock options" thread, I found that all I needed was about 1/4 to 3/8" space to keep my BCG from making contact with my upper receiver.

S&W only addressed using their buffer in an A2 tube and that is how they came up with the appropriate spacer size to do that.

This 1/4 to 3/8" space is probably the difference in the stroke of the action between RRA (longer stroke, shorter buffers) and everyone else (shorter stroke, longer buffers.)

If we can prove the more "standard" (though there is no such thing) AR-10/308 AR rifle buffer works in these guns, I think it will be a benefit for many owners. The rifle buffers are generally regarded to have superior recoil reduction effects. There are two components to AR recoil, the initial impulse (what all guns have) and the BCG/buffer coming to a stop at it's rearmost travel. Buffers can have a big effect on felt recoil due to this, since an improperly weighted buffer will result in the BCG/buffer slamming the back of the receiver extension harder.

One more thing. If a DPMS LR-308 rifle buffer works, a DPMS LR-308 carbine buffer (2.5" vs 3.25" for the S&W buffer), spring, and standard AR-15 carbine length tube should also work.

What this would mean is that for those owners who wish to use something like the Magpul UBR, there is a way it can be done.

ETA: One more thing, if all these things work, that tells me that you'll also be able to use the Vltor A5 receiver extension in lieu of S&W's carbine receiver extension. What this will allow you to do is keep the S&W buffer and spring, and fully collapse a lengthened stock like the Vltor EMOD & Magpul ACS/ACS-L/STR.

Spart, you are over emphasizing the significance of the buffer's role in the whole of the weapons operating system. The buffer is just a catch weight that balances the inertia of the bolt carrier group with the force vectors exerted by the gas system against the opposing force vector of the recoil spring. Additionally, this balance and the operating cycle must occur within the run of the extension tube or the weapon will short cycle, not cycle or hard cycle. A small variance in the balance of the system can cause these issues to occur intermittently without warning or indication of cause. This is why I use an adjustable gas block with my configuration to insure tuning.

If you just feel that you need a heavier buffer, you don't have to buy a buffer from another system ... nor do you need a rifle buffer as a heavier alternative ... just drive out the retaining pin on the carbine buffer end pad and exchange the lighter weights for tungsten weights.

Quite honestly, I say hats off to S&W for devising a simple $10 solution for folks so they don't have to spend $50 for a different buffer and spring when they convert from a carbine to rifle stock. I really have to applaud them for doing this, because they very well could have sent folks down a more complicated path ... or charged them more for a new proprietary buffer & spring even. Now, as far as S&W's communication goes, only time will tell if they can get this information distributed efficiently and effectively.

If a buffer did that and little else, it could be a solid part. But it isn't.

The weights inside have play for a reason, they function in the same way as a dead blow hammer: they distribute force over a longer interval compared to a solid weight.

The effect of this is two-part, cyclic rate reduction as well as rebound reduction, the latter more commonly referred to as bolt bounce. But to just discuss bolt bounce would be inappropriate since the sliding weight has the same effect at the maximum rearward travel.

A reduced cyclic rate is conducive to reliability in that the magazine spring and follower have more time to "catch up" to the cycling action. A rifle with too high a cyclic rate and too little magazine feed pressure will fail to feed.

These are the reasons why the Vltor A5 system is popular in the AR world, the carbine buffer system of the AR being seen as a potential handicap to reliability.

Keep in mind that the "original" buffer system in both the AR15 and AR10/308 AR was the rifle system. The carbine system is a compromise on both.

I am not disagreeing that the gas system must be matched to the complete BCG/buffer/spring system, but springs may be interchanged as well to tune a rifle.

Regarding S&W, they currently are the sole source for spacers and they may stop producing them. More options are better, unless you're just naysaying for effect?

Spart, the 2.5 inch buffer length seems to allow the 4-3/8" of the BCM to protrude into the AR15 mil spec buffer extension sufficiently. Since the spring seat offset is .25" (allowing the spring to coil onto 2.25" of the buffer body), the issue will be the buffer spring and determining a spring of sufficient dynamics that will compress into a minimum 2.25" to maximum 2.37" deformed length. This may be a problem or it may not ... just comes down to the thickness of the wire and how many coils you need to compress. However, as it stands, my attempt to compress the MP10 buffer spring within this limit failed. My attempt to compress the shorter AR15 buffer spring into this limit also failed.

If you use the DPMS style 2.5" carbine buffer, you also need to use the DPMS AR-10 carbine spring. It's a special spring designed to only work with the 2.5" buffer. Other springs will NOT work.

FWIW, we have been fixing these issues since the first M&P 10's came out. There are a lot of threads on this. It is good that S&W watch the forum, and take time to try out the fixes that the members have posted up.
Lead fishing weights are cheaper than tungsten...... and you probably have some in your garage in a tackle box!! But that they responded and put forth a solution is good to hear; the kind of thing one expects from a company like Smith and Wesson.

EDIT; Spart- if you need washers/ spacers, etc. there are a ton of plastic spacers available at most any hardware store. All you need is a tape measure, some imagination, and maybe a file, etc. If you mess one up..... you are out a whopping .13 cents or so.

When we look to solve cycling issues, we first look to the PROBLEM. Specifically, why it is not cycling. Second, we go to the parts and try to get 2 identical units for comparison. This gave me fits, as I have my hands on all manner of rifles, but could not get 2 M&P 10's in the same room to save my life for a long time to compare side by side. So, we checked clearances by swapping parts from DPMS, Knights, LWRC, and Bushmaster. We found several cockeyed combos that made the rifle stone cold dependable. But the key was that the M&P was not far off working properly- so the fix should be pretty simple. In the end, we found there was one buffer which was really close to the same size as the M&P unit, but heavier. And the rifle liked it. So, we just tore open the buffer and swapped out some weight. Problem was solved.

As to the springs, remember that they WILL wear and soften as they are cycled. We were befuddled by this as we found the Knights' rifle had a very well used spring, and gave a super smooth action, as it was worn and the rifle was actually a little sluggish with it (regardless of the rifle it was in, it was smooth but 'lazy'.) We found that the M&P spring/ buffer combo did not work out well within any of the other rifles. Yeah..... it's a little proprietary. OAL not withstanding, when the springs get sacked, they will shorten AFTER cycling. Give them time to relax, and they tend to extend back to nearly original spec length. But run a few rounds thru, and they will prove their loss of elasticity. Compressing a worn spring is a Band-Aid, and the spring will compress more rapidly and continue to degrade. Yes, you can buy a little extra time.
Cutting springs does NOT reduce spring rate. It will project a different feel and felt spring rate because the given length will have changed; a longer spring with a closer coil spacing is usually a softer spring, with a smoother action. Cut said spring, and you get a softer spring because of the spring rate, but the spring will bottom out/ bind early.
A spring of the same diameter coil, with the same diameter wire with a lower number of coils for a given length will tend to have a higher spring rate, and a less smooth action. [More choppy/ short action] Cutting this spring will generally mess with OAL and wind up with a much more harsh spring action. Compressing this spring tends to lead to quicker spring collapsing; a taller spring rate does not respond as well to being compressed in static as well as a lower spring rate on the same given diameter.

Sorry to have just complicated your evening guys....!!!

Spart, if you do decide to take on the expense to try the DPMS LR308carbine spring on the MP10 ... I can only wish you the best of luck ... believe it or not I'm rooting for this breakthrough. However, I actually own both the LR308 and the MP10. The LR308 is over-gassed by design and the spring is tuned dynamically for this condition. Although a different length than the carbine, I have actually tried the DPMS LR-308 rifle spring in the M&P 10 with the DPMS A2 rifle extension tube and DPMS LR-308 rifle buffer ... the M&P 10 short cycled consistently with the DPMS LR-308 rifle spring ... in most cases the bolt did not retract far enough to pull a round from the magazine. Before I test fired that configuration in the MP10, I was actually pretty convinced that it would work because I could draw the bolt back fully and engage the catch ... nevertheless it failed under test fire. Also, in that trial, I observed that the DPMS rifle spring uses a thicker spring wire than the M&P spring. Anyhow, take care and best of luck with your endeavor. I'm hoping you get your hands on some parts soon and prove your hypothesis rightfully ... just be safe.

Luckily the DPMS system isn't the only one out there. I think an ArmaLite buffer and spring may just be the ticket.

Spart, I was able to get the DPMS LR308 rifle spring to cycle yesterday by enlarging my gas port slightly and readjusting the SLR DA7 gas block flow. Hence, the other OEM 2.5" buffer systems can possibly be made to work in your rifle .... just maybe not as the quick and simple "drop-in" solution originally perceived. Before doing this, I think there are some considerations a rifle owner should make before trying(especially if resources are limited to the rifle owner):

1) there are some timing calculations and cycle models that need to be analyzed relative to the current gas port location and determining the gas port size that is needed to cycle the weight of the buffer solution you are considering under the resulting gas pulse length ... especially if you are expecting to achieve rapid rates of fire for whatever the reason.

2) consideration of cost sensitivity for rework or replacement if your port job goes wrong.

I actually returned and readjusted my rifle back to the previous configuration with the lighter S&W carbine spring and the Enidine 308 rifle buffer because it was more comfortable to my shooting style and the purpose of use for my rifle.

I see guys going this far and I wonder......WHY??!!!

The M&P10 is a mid-length gas system. This in and of itself tends to cause a lot of headaches, because people insist on trying to use a carbine this, and a rifle length that. It does not help when a manufacturer decides to wander off the well beaten path and start doing this of its' own accord.

Try to muck with the rifle less, and get more out of it.

Gas system length and buffer type are not mutually exclusive.

I know. And that is WHY the AR10 platform never got real traction. Too many people kept playing with it, trying to make what worked reasonably well immeasurably better. And that is WHY so many run like ****.

I've been dealing with them since ?? 1988. And the most common gripe was always a feeding issue. Always. It's a magazine thing; this one will work in this AR10. But not that AR10. Then, this one will work in a Knights, and an AR10. But not a DPMS. But THAT one will work in a DPMS, and a Knights. But not in an AR10.
And then the gas system........ welllll, it shot just fine. But I wanted it to shoot softer, so I swapped the spring for this. And the buffer for that. And now- it works perfectly, so long as I put a 30-06 powder charge in my 308 cases. SUCCESS!!
Or, it won't shoot anything but brass. I think the metal cases hang it up. So I need to change the gas system up and need to hunt for a buffer and spring combo. But now, it won't cycle. It bottoms out. Or the spring binds. Or the bolt fails to strip the next round.

I have heard each one of these, and seen a lot of fixes on even new rifles.
And in the end, I wonder WHAT it is the shooter was looking for??
A defense rifle of the highest order? If so, it needs to be stone reliable and eat every and any ammo it gets thrown in its' mouth, from Jerry's Jammy hot loads to cheap Wolf ammo, or regular ole hunting ammo and everything in between.
A bench queen? Well, okay, but let's face it: A good bolt rifle will cost 25-70% less, and be superior for that job right out of the box.
Casual plinker? Well, make it shoot cheap ammo, because 308 costs more than 223, and a lot more than 22LR.
Hunting rifle? I'd want the rifle to be lighter than this platform, but to each their own. I'd want to have it be just as durable as the combat version, but with a little bit better trigger, and actually, that is how my particular 308's are set up.

Please understand, I am not picking on you guys or saying you have created the problems described. I just help fix them- and wonder why some folks toss the working mechanism out the window and start juggling gun guts.

Well put.

Sent from my SCH-I605 using Tapatalk

Thanks for your mature attitude on the subject.

This is a thread about customizing the M&P10, if you'd like to worship S&W engineers, start your own thread.

Actually, I felt the S&W team did not get the M&P 10 sorted very well. I spent a lot of time making a couple of them work right.

And YES, there are some threads on it. Some are a year old at this point.

(EDIT) But I get it; you guys are adding a different butt stock on the rifle. And the M&P is a little different than the rest of the 308 AR's.

Which is the part that frustrates everyone; it seems no 2 are alike. Why.... I have no idea.

Spart, getting back to my observations, the gas port diameter for the MP10 is relatively close to the minimum tolerance for an 18" barrel. I did not have my full pin gauge set with me when I ported mine, but I was able to confirm that the port was slightly larger than .069" and somewhat smaller than .078". I ported my gas port to almost .080". This setting works harmoniously with my preferred component selection (as prior discussed) and also cycled with the beefier DPMS 308 rifle spring in testing yesterday. BTW, in a 30 second timed exercise today, the rifle expedited 20 rounds (full magazine) into a 3 inch group at 100 yards from a bipod prone position using CBC 168 grain BTHP. Anyhow, I hope that you see your vision through. I am very glad that you chose to vet your ideas in this forum.

That's good to hear. Just because S&W chose to do it one way, doesn't mean we can't tinker. Despite assertions to the contrary.

I'm of the opinion that a rifle that is properly tuned with the rifle buffer will be more robust than one tuned with the carbine buffer, if only because of the enhanced buffering effect giving more time for the magazine to feed and the spent casing to eject.

I am going to purchase both ArmaLite and DPMS rifle springs to compare them.

Well, I CAN say- if you buy one of these rifles, it is in your best interest to make them work perfectly. It might sound silly to a rifleman who is usually a 1 shot and drop hunter, but in the middle of the night, when a spotlight hits a dozen hogs and you put the dot on that first one and squeeze off..... the AR10 platform delivers the fastest 2nd shot of anything with that much hit power, and it steers much better than almost any other rifle due to it's overall centralized weight platform (The 24" barrel and bull barrel negate this....)
It most certainly has its' place. Good luck making them sing for you!!

Here's some info that may help, I installed one of JP's Captured Spring Buffers about 1000 rounds ago and have not had a single problem. Purchased the "Spring Pack" with the "Buffer" thinking I might need to use them for adjustments, but never even took them out of the bag.

Silent Captured Spring (patent pending) for AR-10-type rifles

Includes spacer to allow for installation in both carbine-length and rifle-length buffer tubes.

From JP's Website: JP - Buffer Springs and Silent Captured Springs

Compatibility

The JPSCS-AR10 is designed to work in standard 7-inch M16/M4-length buffer extension tubes on .308 AR-type rifles like the DPMS LR-308. Armalite .308 rifles, likewise, use these standard M16 extension tubes with rifle-length stocks, so the JPSCS-AR10 will function normally when installed with the supplied spacer. However, Armaliteís .308 carbines use a slightly longer, nonstandard carbine extension tube. The JPSCS-AR10 can still be used with these extension tubes by installing the spacer, which must be modified to fit. Alternately, the system will function readily if the longer Armalite carbine tube is replaced with a standard 7-inch .223 carbine extension tube.

The JPSCS-AR10 is not compatible with the Rock River Arms LAR-8 because of its longer bolt stroke and subsequently longer extension tube.

SPRING PACK

The Auxiliary Spring Rate Kit for the JP Silent Captured Spring allow custom tuning of the device to a given rifle. While the default spring installed in the JPSCS is selected to offer the best performance for the widest range of configurations, a simple disassembly and reassembly of the JPSCS with a slightly different spring rate can yield truly optimized performance, particularly for rifles without an adjustable gas block.

In selecting a spring for your rifle, install a heavier spring in you feel the buffer head fully compress and bottom out while firing the weapon. If the bolt carrier will not lock back reliably on the last round fired, even when completely gassed (in the case of an adjustable gas block), select a lighter spring.

On large-frame rifles, a lower spring rate (i.e., heavier spring) with a properly tuned gas system can often yield a faster, lighter impulse that feels almost like an AR-15 in being very easy to clear or cycle for charging. Using the JPSCS-10 without a JP adjustable gas system will likely require a heavier spring to overcome the friction and extra mass in the bolt carrier relationship. Then again, since Armalite rifles use a McFarland gas ring and have a very low bolt/carrier internal friction, they can often strip, feed and go into battery with a lower spring rate. Because of the inevitable variability in platforms, this spring kit is designed to get any rifle cycling successfully with our JPSCS system.

The included springs from lightest to heaviest are listed below with the color with which they are coded:

JPSCS-SPRING10-100: 1 side red
JPSCS-SPRING10-105: 1 side blue and 1 side red
JPSCS-SPRING10-110: 1 side yellow and 1 side red

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Corpsman FMF/SW

I had been wondering about that myself, did you modify the provided spacer per the instructions for ArmaLite setups?