I apologize in advance for the length of this post. My goal with this is to share the evolution of my 9mm SBR build along with some technical information that I hope can help other members out. The AR in a 9mm flavor is a blast but can also be frustrating to get it to run right and reliably. Maybe you can learn from me and save some time, money, and heartache. I want this to be an open discussion on fine tuning these rigs and getting maximum performance out of them.


Background





Once you SBR an AR you may never shoot a 16"+ rifle again. And if you are like me, once you go short (think 10.5" 5.56mm rifle) you constantly push the limits of developing the ultimate PDW/subgun in the smallest form factor possible. A 9mm SBR AR is a great choice as you can go extremely short on the barrel (5" in my case) without the ballistics being affected too much. 9mm is cheaper to shoot than 5.56 and makes for a great suppressor host. Combine this with the modularity and excellent ergonomics from the AR platform and you have a real winner.




Concept/Goal




I wanted to build the smallest 9mm SBR possible that was reliable (both suppressed and unsuppressed), fun to shoot, looked good (subjective to the individual), and ultimately satisfied my desire to own a "subgun/PDW."




Thus the first iteration was born (strongly influenced by carbonfly's build):









Specs:


QC10 GSF Lower


VLTOR Upper


Adco 5" Barrel


JP 9mm Bolt


UCIW Stock w/ Slash U9 Buffer and Spring


Chopped Wilson TRIM Rail


Wilson VFG


KAC URX Handstop




I chose the QC10 GSF lower as I am a Glock 19 owner. I have my fair share of Glock magazines and liked the idea of a single magazine ecosystem fitting multiple firearms. Plus factory Glock mags are well made and there are tons of aftermarket options out there with new ones hitting the market (see Magpul). The JP bolt comes ramped and cut for Glock magazines so it was a natural choice. I'm not a KNS pin fan but given how hard the blowback nature is of the 9mm AR I decided to use them as cheap insurance for the FCG. The VFG and handstop give a consistent gripping point and made sure I didn't put my fingers in front of the barrel.  The UCIW stock is also a great option for these builds as it keeps the length as short as possible while offering a nice cheekweld and locks up tightly. Here's a comparison (not my picture) of the UCIW buffer tube vs. a standard milspec tube to show the  savings on length.




 




Iteration 2



















I quickly realized that although the HK 416 sights look great and shoot great on this rifle I really prefer RDS optics. I swapped them out for the Aimpoint R1 (discontinued version of the H1) for the small form factor. I chose the KAC NVG mount to give me a "heads up" position as it can be very uncomfortable to get hunched up on such a small weapon to get a sight picture with mounts at normal heights. My Mystic X suppressor finally cleared and it was time to really put this build to the test.  




Challenges with this Build




Let me first say that this rifle has run great from day 1. No jams or malfunctions to speak of. My biggest gripe as many GSF lower owners are familiar with, was how the last round bolt hold open (LRBHO) did not function. See below (not my picture) if you are not familiar with how the Glock lower bolt catch works. One advantage the Colt lowers have is a very reliable bolt catch. How big of a deal is this? Plenty of weapon systems do not feature LRBHO so I figured I could live with it. However, as time went on I grew frustrated with finding out my mag was out with the "click" on an empty chamber. As a huge AR fan I enjoy the feedback "push" of feeling the bolt lock back on an empty mag. I wanted to get the bottom of the issue.    




9mm being blowback operated means a balance of mass (also true in DI). Am I short stroking or getting bolt bounce? You don't want extraction to occur too quickly but also you don't want failures to extract. Add a suppressor to the mix and the equation changes from the additional backpressure. I could insert an empty mag, pull the charging handle back, and the bolt would catch every time. That was a good sign. No dice however when firing unsuppressed/suppressed. A member here suggested holding the bolt catch when firing to see if it worked. Indeed it did. This confirmed his suspicion of bolt bounce (cycling too fast) where it appeared the magazine follower was lagging behind the bolt and never had a chance to engage.




 

Very cool, thanks for both the pictures and your rationale for your choices.

With the QC10 bolt catch spring and a genuine Glock magazine, most people should have decent BHO function, as long as they keep the transfer bar (and its little home in the lower) clean.  With your very short barrel and suppressor added to the mix, you had everything set to give you bolt bounce, or bolt speed too high for the magazine spring to accommodate.  That 12 oz buffer must make the gun feel a lot different from the way it did your original, relatively light 9mm buffer!

Thanks for the info OP!

Nice write up. Due to the rain out here, haven't had time to try a new buffer & spring combo. I made a buffer from some brass stock 6oz and installed a rifle length spring. My 9mm, ddles lower, GM (10.5") bbl & cmmg (modified) BCG had no issues running a H6 buffer / stock spring. BUT want to see what some weight and heavier spring may or may not do, when running suppressed.
If i want to go real heavy. I have the my JRC buffer (12oz) and spring to road test.

Great write-up and info!!  Let me add that I run the Slash 9Q-T/Wolf XP-S and my LRBHO functions 100%.  I also only shoot suppressed.  One thing I noticed on GSF lowers is that there is a little washer (on the dowel pin) that goes between the frame and the Bolt hold open bar.  Make sure your washer came installed towards the "outside" of the frame.  Ive seen a couple that came with the washer on the inside and was almost impossible to make the LRBHO work.  Good luck with your continued evolution Rusell....

Based on what I've seen I'd venture to say more GSF owners have problems with the BHO than not. I do not know if it's due to bolt bounce or the transfer bar itself. I've heard (not sure if correctly) that QC10 is working on a solution to the lack of the GSF reliability. It'll be interesting to see what they come up with and if it is something that can be retrofitted to current GSF lowers. The 12oz buffer is stout for sure! People are always amazed at the density of the 9mm platform. They look at the form factor and assume it's lightweight. The BCG and buffer make for heavier than expected setup and recoil is a bit surprising due to the blowback nature and all that reciprocating mass.



I love that setup and look forward to transforming mine into something similar. I'm happy to hear that the 9Q-T is working well for you. I'm looking to put mine through its paces in the near future. Thanks for the washer tip. I'll have to look and confirm the setup on mine.

Awesome write up!
Thanks for the info. I have a similar setup with and octane 9. I have yet to take it to the range.
So... this gives me some great info.
Just curious if you have some pics of your current setup with a normal carbine buffer tube?

 
Here are some pictures as it is in transition. Normal receiver extension and CTR stock installed now. I'm going to try a CAR stock out as a small option on the standard extension tube.




















And a mock up of how the SD handguard will sit with the APEX handguard when it comes in and is installed.







 
 

Great post. I too am a fan of the SD look. Looking forward to more updates.

Nice.
Looks good with the CTR. It'll be hard to give up the look of the ultra compact stock but it would be worth it for operational BHO.
Looking forward to your updates.
Thanks again!

Thanks for the information, I wish I would have known this before I sprung for the UCIW and Slash's buffer though I haven't got a chance to test it yet but I would like to get the LRBHO running reliably so I may just have to sink some more money into the build.

 
Are you using a GSF or Colt lower? Let me know how it runs when you get the chance.

I'm interested to see what your opinion of the buffer insert is when you get it working. It looks like there's very little to fail on it, but the paranoid part of my brain tells me adding doo-dads to the BCG is a bad idea.
I'd like to try one on my 10.3" 5.56, it's a bit overgassed still.

Thank you to everyone who has contributed to this, it has helped me out so much.

 
I should try the carrier weight system on my 10.5" 5.56. I swear you must be able to drive a truck through the gas port. I've got an H3 in there right now and it still may be overgassed. When my rifle suppressor clears I will certainly give it another look. An adjustable gas block may be in my future...


 

QC10 Just posted their new Bolt Hold Open design. It fits underneath the Bolt catch to allow better lifting action. Maybe you should try it out.

http://www.quartercircle10.com/product/last-round-bolt-hold-open/

I'll have to get one and see.  Looks to be of thicker material .
Too bad it isn't a free upgrade for those with the old style lever...

I called them about the new design and apparently they have been on the included on their lowers for over a year.

 
Is it truly a new design or is this just an opportunity to purchase a spare one (not sure if they've offered them in the past)? I'll have to take a close look and see how it compares to my currently installed one.



 


ETA: I think I see it now. Instead of the bolt catch fitting inside the "square" on the actuator it appears it goes underneath the bolt catch and lifts it up (look at the lower picture in my first post). I guess I'll purchase one and see if it improves.

Here you go: (levers aren't in same orientation)

I'd appreciate it if you could try it and let me know what you think. I've got a FSB 10.3" that is ok unsuppressed, but is going to wear a suppressor soon.

Just wanted to show a stock comparison of the UCIW, CAR, and CTR. The CAR is pretty minimal for a normal receiver extension.

Ran my QC10 Glock SF today and I noticed my ejected brass seemed a little bulged. What does this mean?

All mine is as well.  QC10 5" barrel

What ammo are you using? Sometimes this could indicate early extraction before chamber pressure has dropped to a more appropriate level. What buffers/springs are you guys using?

 

Ammo was "Perfecta" which is a VERY soft shooting 9mm 115gr carried by Walmart made by Fiocchi.

Buffer and springs are JP silent springs.

I was using a wolf Xp spring with a slash heavy buffer set at 9Q.

Most 9mm bolts come with the tungsten weight already present.  You might try finding one that has it, that should allow you to keep the UCIW.

MFT Minimalist looks right at home on a 9mm PDW in my opinion.

Thank you for your post RA. Not too long at all. In fact I'd have kept reading if it were 3x as long.

Your 9mm build is a huge inspiration for my Stag LH PCC and your dri-power 360 are the only T-shirts I wear for undershirts.

There are several threads, but multiple users report buldged brass, shavings and even half moon circles around spent brass.  

From what other users have reported usually QC10 barrels are better than most in this area due to the smoothed feeding sections around the barrel.  Macon Armory is helping diagnose something similar on another thread as well.

http://www.ar15.com/forums/t_3_15/676482_Franken_9mm_AR_FTF_problems.html

I was using a QC barrel

Greetings,

Thanks much for the link to the new lever! I just ordered one. I've not been able to get my DDLES GSF 9mm to work reliably in regard to last round bolt hold open. I'll report back after it arrives and I install it.  Curious if Rudy (MM) had any input into the new part; he was supposed to be working on an improvement. BTW, nice carbine OP and thanks for the thread!

Regards, Jim

My lower that came with that new "improved" bar seems to not work at all. I tried 2 different 33 round Glock factory mags and 1 Magpul 17 round mag (after a little modding) but none will even come close to lifting the bolt catch enough to work.

Same here.

I've read that swapping out the Glock mag springs for Wolf springs makes a difference. I haven't tried it, but I plan to give it a shot.

I almost swapped out my sings but I wasn't sure if the colt springs would work in the Glock. Missed the DSG sale.

Great looking sbr.  Did you look at the JP silent spring at all?  I watched a YouTube video of a guy with a similar set up, and he was running one with the LWRC compact stock.