Buffer spacers in an AR 10, .308

gun'sRgood · Feb 22, 2021

I'm still trying to get this gun running. Yesterday my Wife found the box for the stock and it says no buffer spacer is needed. I felt doubtful and took a buffer and stuck it in the tube then the spring and weight. This is the best the gun has run. Two rounds, It stripped and shot both then did not put the third into battery. Here's some pic. I think the smartest guys with the most experience tend to hang around this forum. So if ya got a moment. I'd sure appreciate your time.

Gordon · Feb 22, 2021

AR 10 s with a carbine stock require a AR10 buffer and spring. The AR 10 buffer is shorter than the standard buffer, but I have found heavier is better. The usual problem with an M5 Aero set up as yours appears to be in ..308 is over gassing which can readily be controlled by an adjustable gas block. I also run a Heavyweight AR10 buffer in mine. In 6.5 creed more they seem less fussy about gas, maybe recoil impulse being less. I went to a rifle stock and rifle buffer and the problems went away for my heavy 22" 6.5 cm M5 build. For my lighter M5 .308 build with the carbine stock it took an adjustable gas block. Un less you have a longer than stock Vltor buffer tube ect. You do need an AR10 buffer length in a Carbine stock. Heavier seems better in .308

Gordon · Feb 22, 2021

I may be wrong in this but my guns now run flawlessly, I was dissatisfied with the first .308 build until I used an adjustable gas block

gun'sRgood · Feb 22, 2021

Were you looking for more or less gas? What symptoms were you running into? Thanks Mr. G!

ColoradoMinuteMan · Feb 22, 2021

Can you explain what you're talking about when you say buffer spacer? You mention stock and buffer spacer both which is what I'm not sure I understand. Some stocks require a buttstock spacer outside fo the a rifle length receiver extension to properly fit the stock. Also there are buffer weights and spacers that people use along with the buffer inside of the receiver extension to tune the impulse cycle. These types of spacers don't have anything to do with the stock. Can you tell us more about the spacer and what is the intended purpose of it?

rbernie · Feb 22, 2021

gun'sRgood said:
I felt doubtful and took a buffer and stuck it in the tube then the spring and weight

Just to be clear : the spring goes in the tube first, and the buffer goes in, all retained by the retainer pin. The buffer face must be pressing against the retaining pin. When the upper and lower are assembled, the buffer is directly behind the bolt carrier group and reciprocates with the bolt, pressing against the spring. The rifle must have all of these parts, and they must be assembled in that order.

Now - there are different weight and length of buffers, just as there are with the spring. You need to make sure that the spring and the buffer are specifically for a large format AR (LFAR, aka an AR10, aka LR308) and not an AR-15. You also need to make sure that you use carbine buffers / springs with carbine buffer tubes (receiver extension, technically) and rifle buffers/springs with rifle buffer tube / receiver extension.

MySuomi · Feb 22, 2021

I have built 2 AR10's. I purchased a 308 spring and buffer for both. Used an Aero lower on mine as well with a Alexandria arms upper on one and a PSA on the other. The buffer tube was a carbine length on both. The buffer tube was not 308 specific. Only the spring and buffer were labelled for the 308. No spacers or anything else in mine other than spring/buffer.

rbernie · Feb 22, 2021

MySuomi said:
The buffer tube was a carbine length on both. The buffer tube was not 308 specific. Only the spring and buffer were labelled for the 308.

Excellent - that sounds correct.

MySuomi said:
No spacers or anything else in mine other than spring/buffer.

Buffer spacers are used to restrict travel for blowback/pistol chambered AR15s. AFAIK, they have no other use.

It sounds like you're trying to troubleshoot an issue - can you describe the functional problem and the context around it?

MySuomi · Feb 22, 2021

rbernie said:
Excellent - that sounds correct.

Buffer spacers are used to restrict travel for blowback/pistol chambered AR15s. AFAIK, they have no other use.

It sounds like you're trying to troubleshoot an issue - can you describe the functional problem and the context around it?

Hey Rbernie, I was only trying to tell the OP what my experience was. Both of my 308 builds are working fine and have been since I put them together.

brasscollector · Feb 22, 2021

When the box/instructions for the stock mentioned no spacer needed I'm sure they were referring to an "A2 stock spacer" which is needed to fit an A2 stock onto a rifle-length buffer tube. Those spacers go on the outside of the buffer tube, between the tube and the stock (inside the stock). They do not go inside the tube and have nothing to do with the function of the gun other than correctly fitting the stock to the tube. The magpul rifle stocks fit to the tubes without any spacers.

Did you use a rifle length buffer tube on the green one?
If you have a rifle length buffer tube I'd suggest getting a rifle buffer & spring and not some spacer that allows the use of a carbine buffer and that would be my advice for an AR15. Knowing you're dealing with AR10s I am even more strict in my advice to stick to the basics and using one brand for parts. Trying to use a rifle length buffer tube, with a buffer spacer and a carbine buffer & spring is asking for problems IMO. Rifle length buffer tube = rifle buffer and spring, carbine buffer tube = carbine buffer and spring.

For your setups there shouldn't be anything inside the buffer tubes except the correct buffer/spring. Without digging through the safe and tearing them apart I can say that the 308 carbine buffers are shorter than an ar15 buffer by about 1/2" and the Aero ones weigh in @ 3.8oz. If you're using a rifle-length buffer tube you need a rifle buffer which is completely different than a carbine buffer in weight, length and overall design (a rifle buffer weighs over 5oz).

brasscollector · Feb 22, 2021

rbernie said:
Buffer spacers are used to restrict travel for blowback/pistol chambered AR15s. AFAIK, they have no other use

Spikes (and likely a handful of others) makes a spacer for AR15s that allows the use of a 223/5.56 carbine buffer/spring inside a rifle length buffer tube. Why a person would actually need one of those is beyond me, especially since they go for about the same price as what it would cost to just buy the correct rifle buffer & spring anyway. I can see using a spacer for pistol caliber conversions.

Navy87Guy · Feb 22, 2021

rbernie said:
Just to be clear : the spring goes in the tube first, and the buffer goes in, all retained by the retainer pin. The buffer face must be pressing against the retaining pin. When the upper and lower are assembled, the buffer is directly behind the bolt carrier group and reciprocates with the bolt, pressing against the spring. The rifle must have all of these parts, and they must be assembled in that order.

^^^ This! @gun'sRgood - If your original post was accurate, try reassembling as described above.

gun'sRgood · Feb 22, 2021

rbernie said:
Just to be clear : the spring goes in the tube first, and the buffer goes in, all retained by the retainer pin. The buffer face must be pressing against the retaining pin. When the upper and lower are assembled, the buffer is directly behind the bolt carrier group and reciprocates with the bolt, pressing against the spring. The rifle must have all of these parts, and they must be assembled in that order.

Now - there are different weight and length of buffers, just as there are with the spring. You need to make sure that the spring and the buffer are specifically for a large format AR (LFAR, aka an AR10, aka LR308) and not an AR-15. You also need to make sure that you use carbine buffers / springs with carbine buffer tubes (receiver extension, technically) and rifle buffers/springs with rifle buffer tube / receiver extension.

Yes sir. I'm learning more and more about the differences. It's been totally worth all the grief. The folks putting up with me have all been fantastic! You are amongst them. Thanks for the mentoring. Never had a dad or older brother to learn anything from. Closest help is my dear Wife telling me stories of how she used to help her Dad some thirty years ago. He has passed way before I got to meet him. So Thanks and Thanks for your service! You guys are the best!

gun'sRgood · Feb 22, 2021

brasscollector said:
When the box/instructions for the stock mentioned no spacer needed I'm sure they were referring to an "A2 stock spacer" which is needed to fit an A2 stock onto a rifle-length buffer tube. Those spacers go on the outside of the buffer tube, between the tube and the stock (inside the stock). They do not go inside the tube and have nothing to do with the function of the gun other than correctly fitting the stock to the tube. The magpul rifle stocks fit to the tubes without any spacers.

Did you use a rifle length buffer tube on the green one?
If you have a rifle length buffer tube I'd suggest getting a rifle buffer & spring and not some spacer that allows the use of a carbine buffer and that would be my advice for an AR15. Knowing you're dealing with AR10s I am even more strict in my advice to stick to the basics and using one brand for parts. Trying to use a rifle length buffer tube, with a buffer spacer and a carbine buffer & spring is asking for problems IMO. Rifle length buffer tube = rifle buffer and spring, carbine buffer tube = carbine buffer and spring.

For your setups there shouldn't be anything inside the buffer tubes except the correct buffer/spring. Without digging through the safe and tearing them apart I can say that the 308 carbine buffers are shorter than an ar15 buffer by about 1/2" and the Aero ones weigh in @ 3.8oz. If you're using a rifle-length buffer tube you need a rifle buffer which is completely different than a carbine buffer in weight, length and overall design (a rifle buffer weighs over 5oz).

Thanks! I think your spot on. I've ordered some other parts from the suggestions here. This has been just a wonderful learning experience! The $$$ is not an issue nor is time. This build is for a good friend and I believe he is enjoying watching me struggle. I will get it right and I'll owe a bunch of you all the BBQ and Beer you want. I'm retired and thanks to you all, I'm having the time of my life. [I am going to have to reload a few .308 though. This R&D takes a few rounds!]

LoonWulf · Feb 22, 2021

Is it just me seeing stuff, or is the buffer retaining pin on your silver one sheared off?

Charlie98 · Feb 22, 2021

Gordon said:
AR 10 s with a carbine stock require a AR10 buffer and spring. The AR 10 buffer is shorter than the standard buffer, but I have found heavier is better. The usual problem with an M5 Aero set up as yours appears to be in ..308 is over gassing which can readily be controlled by an adjustable gas block. I also run a Heavyweight buffer in mine. In 6.5 creed more they seem less fussy about gas, maybe recoil impulse being less. I went to a rifle stock and the problems went away for my heavy 22" 6.5 cm M5 build. For my lighter M5 .308 build with the carbine stock it took an adjustable gas block. Un less you have a longer than stock Vltor buffer tube ect. You do need an AR10 buffer length in a Carbine stock. Heavier seems better in .308

Funny, that mirrors almost exactly my experiences.

My first AR-10 build, a mid-length .308 with a carbine stock, is insanely overgassed. I used the stock carbine buffer and spring. In hindsight, I wish I would have sprung for an adjustable gas block, and I may retrofit it, eventually. I will likely go to a heavier buffer as well.

I have a 6.5CM build up on the bench right now... 22" heavy XL rifle barrel. I put an adjustable gas block on it, but the more I research it, the more I find it unlikely I'll need it vs the .308.

brasscollector · Feb 22, 2021

LoonWulf said:
Is it just me seeing stuff, or is the buffer retaining pin on your silver one sheared off?

It also appears there is a shiny ring around the edge of the extension hole which might be an indication the BCG has impacted the lower receiver

.

ColoradoMinuteMan · Feb 22, 2021

I’ve been reading through this post as well as your other posts. It seems like there is a lot going on here and if it were me I would be simplifying things to reduce the number of involved factors. I’d start with determining that you are matching your buffers and receiver extensions. In your pictures I see 2 carbine buffers. The rifle with the PRS stock looks like it has a rifle length receiver extension. Is this true? If it is true you need to use a rifle buffer with that receiver extension. That alone could be your issue but honestly there is a lot going on here and if you are going to solve the issue you need to take a systematic, patient and organized approach.

gun'sRgood · Feb 24, 2021

LoonWulf said:
Is it just me seeing stuff, or is the buffer retaining pin on your silver one sheared off?

Yes! Just another reason I love Black Rain! This people make a crap load of promises that they just never keep. I've since replaced that

Buffer spacers in an AR 10, .308

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