Proper Buffer for 20" Rifle Gas Upper on Carbine Lower?

I want to put a 20" A4 upper on a lower with a carbine receiver extension with a collapsible stock. What is the best buffer for reliability? I've been reading a lot about it but I keep seeing answers all over the board. One respected manufacturer has recommended the standard Carbine buffer when using their 20" rifle gas upper on a carbine RE lower.

I've read that the Canadians with their C7A2 use a "HH" buffer, which I can only take to mean "H2". However, there are many threads of people having problems with H2 buffers and rifle length gas systems.

https://www.m4carbine.net/showthread...7262-H6-Buffer

Edit - the post below addresses carbine length buffers very well.

If you are building the lower, I would suggest an A5 system, which was designed for your stated purpose. I shoot a 18" rifle with an A5 RE, milspec rifle spring, and A5H2 buffer. It works very well with a wide range of ammunition. I changed the barrel from a 20", which ran perfectly with the set up for 2 years.

Andy

My first question is why? Is it to reduce OAL? Do you not have a rifle length lower?

Second question is do you have a carbine lower you can try out? Friend, relative, LGS, etc.? Depending on how it feels and orientation of ejection, I could suggest which way to go for a purchase.

In general, the Carbine H2 buffer is the closest approximation to the standard rifle buffer system, but the spring rate is higher and thus may not work with all ammo pressures (typically on the lower side). You're going to have to test the system to see what works for you. If you have no means to experiment with someone else's setup, you could buy a standard carbine buffer and a H3 (or buy the individual tungsten weights to swap with the steel weights). This way you can tune the weight for the ammo you shoot, the spring you run, and the GP size of the barrel. These variables could be why you are seeing mixed answers.

Well guys I can say I have an 18" (obviously rifle length gas) that I've been running with a collapsible stock and a standard carbine buffer.

Just tripped the 1,000 round mark, it has never malfunctioned with any of the plethora of ammo I've put through it.

Runs and handles fine. I can't see a reason to go heavier.

It's not a one-size-fits-all situation. You could probably go heavier, perhaps much heavier, if you ran a can and/or 5.56 exclusively. Then again, maybe you can't, and that's the point. If you don't have extra buffers laying around, I'd say you're pretty lucky or got good advice.

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Originally Posted by ViniVidivici

Well guys I can say I have an 18" (obviously rifle length gas) that I've been running with a collapsible stock and a standard carbine buffer.

Just tripped the 1,000 round mark, it has never malfunctioned with any of the plethora of ammo I've put through it.

Runs and handles fine. I can't see a reason to go heavier.

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Edited - I ate a snickers and will try again...

It seems to me that folks tend to be negative regarding adjustable stocks on rifle length (18 -20") ARs and I just don't understand it. In my opinion, based on 20+ years shooting M16s, M4s and my own ARs, an adjustable stock makes the AR rifle more suitable for a wider variety of shooters and shooting conditions.

Andy

To the contrary, I appreciated your post. I asked why because IME the 20" barrel paired with a standard RE and buffer is very smooth in operation as well as being one of, if not the most, reliable setups to be had. If OP didn't know that, it should be noted. The mix in question can be, sometimes, painfully finicky and I was hoping to reduce grief from out of shear curiosity of the combo. However, I'll gladly help out even if someone wanted to put a 7" upper on a A2 rifle lower. Yep, seen it done and it ran better than the carbine lower it came from!

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Originally Posted by AndyLate

Edited - did not add anything useful.

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I will build a fixed stock rifle in the future because I like the way they shoot and look. I just won't share it like I do my current ARs.

Andy

The best buffer is an A5, which has equal or better reliability than the rifle buffer, but requires swapping the carbine RE.

In general, the 18-20" uppers are gassed to run a rifle buffer, so that means starting with an H2 or A5H2 weight.

The 20" rifles are pretty forgiving and should run with most reasonable choices.

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Originally Posted by SouthwestAviator

I want to put a 20" A4 upper on a lower with a carbine receiver extension with a collapsible stock. What is the best buffer for reliability? I've been reading a lot about it but I keep seeing answers all over the board.

One respected manufacturer has recommended the standard Carbine buffer when using their 20" rifle gas upper on a carbine RE lower.

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I shot a bunch of steel cased with a 20" BCM using H and carbine buffers without issue.

You can run any properly gassed upper on a lower with an H, H2, A5H2 or rifle buffer (used with the correct RE & spring).

Bumpin' this as I've changed my mind....I'd been running an 18" with a carbine buffer.

Reliability was perfect...but in experimenting with an H2, I have to admit the gun is less jumpy, and runs more smoothly. This became very apparent when running drills like the MNQ.

Still 100% reliable, and runs smoother this way. Will be running it with H2 going forward.

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Originally Posted by ViniVidivici

Bumpin' this as I've changed my mind....I'd been running an 18" with a carbine buffer.

Reliability was perfect...but in experimenting with an H2, I have to admit the gun is less jumpy, and runs more smoothly. This became very apparent when running drills like the MNQ.

Still 100% reliable, and runs smoother this way. Will be running it with H2 going forward.

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I'm slowly coming to believe that H2 is the perfect carbine buffer for everything.

I've used the standard buffer, and the H3. 20" rifle-gas 5.56. They both seem to run just fine.

The H3 buffer is almost the same weight as the rifle buffer, around 5 oz. I believe the standard carbine spring feels a bit stiffer than the rifle spring, but that would make sense with less travel.

Well, maybe buffer travel isn't so different when you consider the longer length of the rifle buffer.

The Army conversion kit to put a collapsible stock on a 20 inch barrel rifle, uses a standard carbine spring and an "H6" buffer.

An H6 buffer is nothing but an H3 buffer with the weights cut to half the original length and twice as many of them. The weight for the H6 and H3 is the same, the only reason the Army uses the H6 is it works better in full auto.

H2 IIRC is what the Canadians used.

Yeah, I've also upgraded an 11.5" to sn H2. Have run it with an H for the last several hundred rounds, and it's also been 100% reliable, but it too is smoother with the H2.

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Originally Posted by MontanaMarine

I've used the standard buffer, and the H3. 20" rifle-gas 5.56. They both seem to run just fine.

The H3 buffer is almost the same weight as the rifle buffer, around 5 oz. I believe the standard carbine spring feels a bit stiffer than the rifle spring, but that would make sense with less travel.

Well, maybe buffer travel isn't so different when you consider the longer length of the rifle buffer.

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Same travel as the bolt stroke is the same.
The difference is the compression % of the spring. The rifle is more uniform/flat, the carbine increases resistance with travel more.