I  got inspired last year to build an AR9, specifically one that used Colt mags and had a full sized magwell, so the receiver looked just like a standard AR15 receiver. That leaves a few quality options in Colt, CMMG, and Spikes (maybe a few others). I ordered my CMMG and have been very happy with it. The decision to build the AR9 was based on two things: how much I like the AR15 platform and how much I like my SBR Scorpion 9MM. So of course once I drifted through the interwebs, I decided to build two different uppers. One super short and one intermediate length. I have since built a 3rd, 16" upper. I found plenty of inspiration for my build in this thread: https://www.ar15.com/forums/ar-15/9mm-AR-gun-porn-ok-lets-see-them-/15-208426/

Specifically, I found this picture:


Perfection: short and compact but very functional. So I ordered some parts and started my build. As part of the build, I wanted to use LWRC’s Ultra Compact Stock Kit. It has a 6” buffer tube and is a fan favorite among the SBR crowd. Plus the entire kit can be had for $100, which includes the buffer tube, the stock, and a spring and buffer. Below you can see its size and how small it is compared to a Moe stock. Here's a good thread comparing some stock lengths for PDW style setups. https://www.ar15.com/forums/armory/-/51-444867/&page=1#i4322198

The problem was that running this combo with a blowback AR9 meant a lot of energy moving to the rear. So I'd need to source a super heavy buffer to keep the bolt from bottoming the action out once it reached full travel. This would impact both function and longevity of the setup, not to mention the sound. My Google-Fu turned up one solution, and that is Slash’s heavy buffer specifically designed for an AR9 using the LWRC ultra compact. People run it and love it. There is a lot of praise for it on here.

Problem: the buffer and spring cost $150. That’s crazy to me for a super short, heavy weight with a spring. http://heavybuffers.com/uciw9.html

So after talking with a few local guys about how much they loves the JP Silent Captured Setup in an AR9, I did more searching. I found a thread where one of the .300 BO guys wanted to run the JP in the LWRC UC, but it was too long. So he called JP and worked with them to buy and cut his own shorter metal rod, ordered new weights and spring combos, and dialed it in.

Naw bruh, I ain’t got time for that. Plus that’d be like $300 by the time you bought one setup and extra weights and springs. Negative.

Fortunately, my persistent Google-Fu turned up the JP Silent Capture setup for the Maxim CQB stock. Maxim sells their CQB combos with the JP SCS and they actually have a dedicated 9mm kit! Even better, the JP SCS for the Maxim CQB in 9mm IS SOLD SEPARATELY!!! PRAISE THE LORT!

So by this time I have the LWRC stock kit and I email JP asking them about the fit of that part into the LWRC kit. I get a quick response from a sales guy who said he talked to one of the engineers and it wouldn’t work. WTF?!?!?! So I said to hell with it, I will do my own math. I measured the inside of the LWRC tube once installed on the CMMG receiver to see the depth to the buffer retainer. I found the JP SCS for the Maxim CQB on Brownell’s website and submitted a question, asking them to measure the overall length. Something like 4 business days later, they responded with the measurements. Gotta love Brownells. Even better, IT WOULD FIT. But… it’d actually be on the short side. No worries, that would be easier to solve than it being too long.

So I ordered the JP SCS for the Maxim, it comes, and IT FITS! I have to add 4 or 5 quarters to get it to the right length, but no worries there. I removed the weight out of the back of the CMMG bcg, which is required for the rod of the JP setup to travel into the rear of the BCG. And then I tested it.

It went boom. Every. Damn. Time. I tried it with my 4.5” upper. Boom. I tried it with my 8.5” upper. Boom. Not only did it go boom, but it ran super smooth with almost no muzzle rise even with mag dumps. I was so happy. I eventually found a nylon washer (of which I had to order 100 off Amazon, but only at $7) to replace the quarters, because you know, ounces equal pounds! To confirm, 1” washers WILL NOT FIT unless you plan on trimming them.

Even better, with my 4.5” upper, the gun is under 18” fully collapsed. That’s more #winning than Charlie Sheen circa 2011. I love the way it turned out.





TL;DR version

If you’d like to run the LWRC Ultra Compact Stock Kit on an AR9 with a JP Silent Captured Setup, here is what you’ll need:

• LWRC Ultra Compact Stock Kit: UPC 859890005851, can be had for under $100 shipped. I used BuckSniper.com and made an offer of I believe $90 which they accepted, so it was like $98 shipped.


• JP Silent Capture Setup for the Maxim CQB Stock, 9mm variant: UPC 8523976120, I found it cheapest at Lanbos, about $152 at the time of this posting. It should read: “MAXIM CQB BUFFER JP SCS-SX GEN2 9MM”


• Either a few quarters or nylon spacers to drop into the back of the receiver extension/buffer tube to space the JP SCS appropriately and keep it from sliding back and forth. I ordered a pack from Amazon called “Uxcell a15061900ux0377 Nylon Flat Insulating Washers Gasket 24 mm x 12 mm x 2 mm 50 Piece Beige, 0.47" Width, 0.94" Length” for like $9


• A 9MM BCG that has a removable rear weight like the CMMG, Faxon, Brownells, etc.

I like it!

Tagged.  Did you ever consider a .308 buffer instead of the silent capture setup?

I've got the Maxim setup, but I don't want to pay out the money for a JP silent capture spring

Nice write-up!

I also went with the LWRC Ultra Compact stock on my AR9.  My solution is to use a standard or extra-power carbine action spring combined with a Colt (or similar pattern) 9mm carbine buffer with the delrin bumper trimmed down to fit the shorter extension.  Works great.

Right or wrong, I did the JP setup because I figured it gave me the best chance to run well with the 3 different uppers and then with a can (Octane 45 pending if SiCo.will.get their shit together, Omega 9K hopefully out of jail in February)

Time will tell if it requires more tuning, but I figure not on a blowback setup.

I found that photo before I built my AR9 as well.

I wonder how many builds it has influenced.

I went to slash route and mine has been 100% since day one. Damn heavy recoil spring though.

I figured there'd be more interest in this! Maybe folks are just reading it and bookmarking it for future use!?!?!

Good write up. That same build inspired me years ago to take my first stab at a 9mm build. I've had a love hate relationship with it ever since just because it's been one of the more finicky builds. The biggest problem with 9mm builds is they are typically direct blowback nature and the amount of mass you have slinging back and forth in a bolt and buffer is significant. When I ran suppressed I was using a 12oz bolt which was just ridiculous. I've recently overhauled my build with CMMG's radial blowback bolt/barrel combination and it has made for a much more forgiving setup. Amphibian is also been a great resource for the tinkering of those systems.

My first iteration



Current setup