I was curious if anyone was running a setup like this.

Do the same theoretical rules of 21-24 OZ between the bolt carrier and buffer weight still apply?

In this solar system, the known rules of math, physics, chemistry, fluid dynamics, and the like apply.  Within and/or near black holes, there is more to be learned.  How one applies the known rules of math, physics, chemistry, fluid dynamics, and the like is sometimes a matter of discussion and/or debate; especially with regard as to what is "best".

Best of luck.
MHO, YMMV, etc.  Be well.

Cool.

From amphibians suggestions (check out his site https://c3junkie.com)
My blowback 9mm has carbine tube, kynshot RB5015 buffer, and tubbs 556 flatwire spring. Very smooth cycling.

My 9mm cmmg RDB also A5 tube, tubbs 556 flatware spring, but uses kynshot RB5007 buffer. Also very smooth.

Are you using a 9mm buffer spacer with that?

My thinking is that if you use a carbine tube with a carbine buffer and need a spacer, you will also need the spacer with the A5 tube and A5 buffer.

It works. Borrowed my A5 buffer for my first ar9 build to try out till I ordered a 9mm buffer. The Taccom PCC is whats in mine now.

I've seen this question a bunch of times, so I put together an article about how it. Here you go:  Vltor A5 parts and 9mm blowback

A5 Buffer? Yes.  A5 buffer tube? No.

The problem with using the A5 buffer tube is rearward overtravel from the shorter 9mm bolt that can break the bolt catch or cause brass to get jammed in the lower's trigger pocket.

From the article:

Carbine tube (7?) + carbine buffer (3.25?) + 5.56 bolt = correct travel. ?
Carbine tube (7?) + carbine buffer (3.25?) + 9mm bolt = 0.75? overtravel. (bad)
Carbine tube (7?) + carbine buffer (3.25?) + 9mm bolt + 0.75? buffer spacer added to the back of the spring = correct travel. ?
Carbine tube (7?) + A5 or Extended buffer (4?) + 9mm bolt = correct travel. ?
A5 tube (7.75?) + A5 or Extended buffer (4?) + 5.56 bolt = correct travel. ?
A5 tube (7.75?) + A5 or Extended buffer (4?) + 9mm bolt = 0.75? overtravel. (bad)
A5 tube (7.75?) + A5 or Extended buffer (4?) + 9mm bolt + 0.75? buffer spacer added to the back of the spring = correct travel. ?

Yes, the same weight rules apply.

Depends if you're going to use a direct blowback or CMMG's RDB system.  The RDB bolt is longer than the blowback bolts I have.  So I don't need a spacer using amphibian's suggested CMMG RDB A5 setup.  

If you're going to use the A5 tube with a shorter direct blowback bolt and barrel, you'll probably need a spacer.  However, if you use a regular carbine buffer tube and direct blowback bolt/barrel, you don't need a space since the Blitzkrieg hydraulic buffer shown (same as the kynshot I think) is longer than a carbine buffer.

Some sources recommend a combined bolt/buffer weight of 21 to 23 ounces for a direct blowback AR9.  A lot of folks have gotten away using less without any problems.  Both my direct blow back and CMMG RDB bolts plus hydraulic buffers have a combined weight of 13 ounces.  A hydraulic buffer seems to reduce the need for a heavier buffer.

I've included a picture to give you an idea of the different systems.  The top is a direct blowback AR9 16" with a carbine buffer tube.  I use a Blitzkrieg RB5015HD hydraulic buffer with a regular carbine spring - runs smooth and and locks back on an empty mag every time.  It also shows a 9MM extended buffer and a regular carbine buffer with 12 quarters extension for comparison to the hydraulic buffer.

The bottom lower is the RDB A5 setup.  

At the bottom of picture, the shorter spring is a regular carbine spring used in the direct blowback and the longer one is the Tubbs flat wound spring in the RDB A5 setup.  

My ar9 with a carbine tube +a5 buffer (all tungsten weights) runs great with super reasonable recoil.

This is great info to hold onto.  I will also say that the Kak extended buffers have sliding weights - which you will need if you decide to use a faster trigger system (binary or forced reset).  You can use a heavy Carbine length buffer (3.25") but I usually add $2.25 in quarters behind the flatwire spring when I switch to 9mm since I only have one extended PCC buffer but two 9mm uppers.  I can put the other upper on a milspec lower and use Endomags (which I love).

I use an A5H4 buffer with carbine tube and carbine spring. The buffer weights 6.8 oz and is the same length as a dedicated/extend 9mm buffer. Works great. My bolt is a Colt.

I was tired when I typed that. Meant to say carbine tube, not A5 tube on the blowback AR lower. I corrected my original post.

OP....quite a few previous threads discussing this already.


https://www.google.com/ar15.com+9mm+a5+buffer




.

Thanks for the information.

I'm mainly curious if I use an A5 tube, and A5 buffer, and a spikes 9MM spacer if it will work.

From what I am gathering it will, but I was curious if anyone was doing that.

I really don't feel like removing the A5 buffer tube.

EDIT: Droppoint actually had it as the last one on the list. It should be good to go using the A5 Tube, A5 Buffer, and Spacer.

Why would you want longer buffer travel?

Almost everyone running PCCs competitively short strokes them. There’s a reason.

Increased buffer (and bolt) travel is desirable if you are trying to reduce cyclic rate in a subgun.  By chopping off the rear of the bolt key and using a carbine-length buffer in an A5 stock, you can reduce cyclic rate by roughly 120 RPM without regard to overall weight of the bolt/buffer combo.  I'm currently running a standard weight bolt (16 oz) with a very heavy buffer (11 oz) and a Tubbs .300 BO spring and the gun is cycling at 500 RPM.  Singles are stupid easy even for an old fart like me.

No, I don't use a LRBHO.