A while back, I decided to sell my MKE SP89 clone.  I didn't shoot it much despite it being reliable, accurate and a close proxy for an MP5k.  Looking around some pic threads (Damn you!) I got the AR-9mm pistol/SBR bug.
Rationalizing it, it seems it would be easier to clean and work on since most parts are standard AR-15 parts and the two halves allow better access than the HK guns.  Also, it's a lot simpler because it's a standard blowback design vs the proprietary and overly complex roller-delayed blowback system of the HK guns.  This isn't meant to bad mouth the MP5 and it's siblings.  They've certainly earned a reputation for being one of the greatest guns ever made.

Now then...

Phase 1: Parts Selection
Upper Half
Colt A4 9mm upper
CMMG 9mm BCG
QC10 Barrel 9mm 5.5" 1/2"-28
BCM Gunfighter Charging Handle Mod. 4
Standard Barrel nut
A1 Flash Hider bored for 9mm

Lower Half
GPI Slim Colt-style Lower
White Oak LPK
ALG ACT trigger
KNS Anti-Rotation Pins
Hahn Precision 9mm Buffer
Carbine spring
MI Pistol RE
Magpul K grip
Magpul Enhanced trigger guard
Magpul ASAP-QD end plate

Pre-assembly parts layout.


Fully assembled lower and hand-fitted upper.

Some parts shown were not used or were swapped out for alternate parts.  I bought a set of "dog bone" KNS pins, but decided the full-sized link would be better.  The DPMS pistol tube hand guard was only used as a place holder.  I chose the...

-GPI lower because it was a less expensive than the QC-10 lowers and I prefer the looks of the Colt-style straight magazine over the angled Glock magazine.  Also, I didn't want to hassle with modifying the BCG despite owning a number of Glock magazines already.

-QC-10 barrel because I wanted a 5.5" barrel with 1/2"-28 threads.  Nearly every barrel I found was 5" with 1/2"-28 or 5.5" with 1/2"-36.  I wanted the 1/2"-28 because when and if I ever get a threaded barrel for my Glock 19, I can use the same silencer.

-Colt A4 9mm upper because I wanted a slick-side upper with just the door.  I figured I'd save a little bit of weight by shedding unnecessary parts as well as give it the ability to have a more streamlined look by removing the brass/gas deflector.

-Hahn Precision 9mm buffer because it's slightly longer so no spacer is needed and it's north of the suggested 5.5oz starting point for 9mm buffers.

Phase 2: Assembly and Test Fire
Assembly of the upper seemed pretty straight forward.  I got some Aeroshell 33MS grease.  Clamped my upper in the vise using some nylon blocks.  I forget who makes them, but they clamp to the bottom and onto the top rail.  I couldn't use a clamshell vise block since the upper has the 9mm deflector and I couldn't use a Geissele reaction rod or the like since the 9mm barrel doesn't have the lugs like a 5.56 barrel.  I did my best with what I had.  I torqued the barrel nut to 30ft/lbs three times.  It needed to turn further since a tooth was just starting to get to the center of the upper where the gas tube hole should be.  I started to torque to around 40 ft/lbs when my armorer's wrench slipped.  The opening was starting to spread.  It appears I bought a cheap wrench.  I don't remember where I bought it, who made it or how much I paid for it. If I had to guess, it is a Tapco special I bought Cabela's or a LGS back around 2008...when I didn't know better.  At this point, I figured the barrel nut is torqued to at least the minimum and it's now capable of being test fired.
Up till now, I haven't mentioned any sort of handguard, sights or what-have-you.  This is because if the gun doesn't work or doesn't work well, I'd rather work to fix it now instead of after I've attached another $300+ worth of parts I may have to remove.

Out of 50rds of Aguila 124gr ball, I only had one stove pipe.  It may have been caused by me.  I had my hand on the magazine and may have wiggled it enough to throw it off.  It could also be that it's still breaking in.


Overall impressions from the first firing are positive.  It does have a bit of muzzle rise, but it's likely due to the heavy buffer and lack of secure grip where the handguard should be.  The cycling action seemed rather slow, as if I could almost hand cycle the bolt faster.  Brass ejection was at 3 o'clock and a bit...lackadaisical.  This and the "slow" action are likely due to the buffer and ammo.  I look forward to finishing this build.

Phase 3: Finishing Touches
I'm trying to decide which rail I want to use.  Since I went with a 5.5" barrel, my rail options are either...
A) one of the 4" rails from Samson, YHM, etc. The problem here is the rail is slightly too short for my liking.
B) Odin Works 5.5" keymod rail. This caught my eye, but it's not quite what I'm looking for.
C) a 7" rail with a 3" muzzle device such as the KAK Flash Can. This may be an interim step, but again, not quite what I had in mind.

This brings me to option D, cut an existing rail down to the desired length.

This is a CAD mock-up of my upper with the Troy Bravo rail.  According to my measurements, I should be able to trim a rail near the #26 rail slot which is usually around the 4.5"-4.7" range.
Other rails I'm considering are the Midwest Industries T-Series (Gen 1 & 2) as well as their K-Series rails.  I was looking at the Troy MRF and the MI equivalent 2-pc free float rail.

As for sights, I'm thinking either the Magpul Pro or the Daniel Defense fixed sights.

What  does the kick feel like compared to a standard AR around 7lbs?

8" ALG M-Lok rail with a xm177 extended Flash Hider until you can get a suppressor...

MAHA

There was no real recoil, but definitely muzzle flip. I wanted to check and make sure the ammo wasn't .380ACP. I'm going to take it out again this weekend with some different ammo.

A suppressor is a long way out. The SBR stamp will be a while too.

DANG!

Just when I was thinking that I had all the SBRs I Need.

Another Want appears!

To the OP.....,

How do you like the GPI Slim Colt-style Lower?

Thanks,

Mark

DMark, the GPI is nicely done. Very solid feeling and good finish.
This whole endeavor started after seeing a gun posted by member 'carbonfly' in the picture thread. (~page 70ish, IIRC.) It's an AR analog to an MP5k PWD...and sexy as hell. Other similar setups by other members solidified the idea. Damn them and my weak will!

Thanks for the info!

Yea, I have a friend who has a MP5k for overseas contract work .  Its a hoot to shoot, but with too many decades behind AR platforms I find the H&K ergonomics to be a bit odd to me.

But I'm liking what you along with Carbonfly/RussellAthletic/ChaptShiess/Tenex have done with these AR 9mm PWDs.

Reckon its time to call my FFL and give him a heads up for another inbound transfer for one of those GPI lowers!

Mark

The AR has the advantage modularity and the capacity of accepting upgrades without requiring the fitment of parts. Uppers an lowers mate, trigger assemblies slide right in, no muss no fuss.

Nice to see you're getting function (issues) if any out of the way before dropping more coin in to it. Myself, i'd buy a longer rail off the EE and cut to size.

Jim, that's precisely what I'm doing. I bought the pieces necessary for the gun to function and figured if it didn't like something, I'd rather replace it now. A pretty paperweight doesn't do me any good.
I was thinking of picking up a used rail off the EE, but I'm looking for specific rails, rails that can be cut without leaving 'voids' from lightening holes. I may do as was suggested and get a longer rail and a muzzle extension for the time being.
As I use and man-handle my new toy, I'm already finding things to change and upgrade. I'm really thinking it needs a 45° (or other non-90°) safety lever. I find myself wanting to hold it with my thumb high like I'm riding the safety on a 1911. Also, the buffer tube has a step which can be a pinch point when operating the charging handle a certain way. We'll see as time goes on.

That GPI lower looks awesome. I really didn't want to spend almost $300 for the QC10 but really wanted one with a smaller magwell. I almost have all of my parts right now and can't wait!

THIS is how my range trip went yesterday.
I tried shooting some Federal 115gr ammo through it and had four "stovepipe" failures out of 25rds. Doing a quick search for "9mm AR15 stovepipe", it sounds like this is an issue with the ejector, but I'd certainly welcome any other suggestions.
I noticed the recoil was a little more pronounced this time around which surprised me as I figured a lighter projectile would mean lighter recoil.  Maybe the Federal is loaded hotter than the Aguila.  It's more likely that I just paid more attention to it.  I'm going to see if I can get my hands on some 147gr stuff and maybe try some WWB.





The basic components list follows:
QC10 5.5" barrel
CMMG bolt carrier
GPI 9mm lower
Hahn 9mm buffer
Std. carbine buffer spring

ejector issues..

Could be ejector, but the slow cycle time sounds odd.
You could be getting a short stroke on the cycle. The original Colt SMG design is based on a 10 1/2" barrel, you are cutting that in half.
You had better luck with the heavier bullet. The lighter bullet would probably have a faster FPS and would leave the barrel quicker. If it's leaving the bore before you are getting a good recoil push on the bolt, it might be short stroking and not getting enough push on the ejector to clear the spent shell.

Try a regular CAR buffer and see how it works.
Also check for ejector drag on bolt.

Also, how are you shooting at the range, one handed like a regular pistol?

I've tried shooting it several ways, one-handed, support hand on mag well, support hand on buffer tube, etc.
I might try a test of 115, 124, & 147gr bullets with the current buffer and with two lighter buffers, another 9mm and an H buffer.
One thing I failed to mention is the magazine I'm using is a Brownells branded Colt mag.

Sounds like you have plan for testing. Also check for last round bolt hold open with each ammo type, could be an indicator of short stroke.
Since you had a good first outing, sounds like you just need a slight adjustment somewhere.

I was asking about the shooting style thinking you might not be giving it enough rear support (no buttstock). Allowing the entire gun to recoil back, taking away a little from the bolt. Easy to check if you're following my direction.

Hope you get it sorted out soon. Nice build!

Thanks Txnrcr!
I tried many different holds, mostly trying to keep the gun as stable as possible.
I just bought a Colt 9mm buffer (5.5oz) and a Wolff extra power spring. I figure I'll try different combos to see which runs best.

I checked the ejector and bolt for signs of wear. Nothing on the bolt, but the ejector had some slight rub marks on one corner. I'll leave it alone for now.

Sooo...
I bought a Colt 9mm buffer and a Wolff extra power spring, gathered a couple of quarters and some ammo of different weights and headed to the range yesterday. I figured I'd load up some 115s and verify the gun didn't like them and guess what...

It ate them all with only one hiccup!

At this point I'm saying "Screw it!" and marching on with the build. I've already got my rail and plan to mount it soon. Today, hopefully.

What did you end up with for a rail?


I also have a 5" barrel and opted for the 3" KAK Flash Can underneath the UTG 7" rail and it barely peeks out using a crush washer (KAK says you don't need one but I needed it for the extra length).




I had similar issues with the extractor:  A small adjustment makes a huge difference.


 

Do you mind taking a pic of how the round sits in the chamber?  I am having issues with mine and would like to compare if I could.  Thanks.

I was talked into the Midwest Industries K-Series Keymod rail.  I was originally thinking about a Troy Bravo or MI Gen-II T-Series.  I've never been really impressed with the look of the Keymod system and I almost tried to change course and use the MI T-Series instead, but I figured "What the heck!"  I had a local gunsmith/machinist trim the rail at the T27 slot because of how it all lined up.  For $30, he did great work.  A little flat barbecue paint was all I needed to finish it.  (Aluma-hide didn't work.) It's actually just a bit longer than I thought it would be, but it will do for now.  Now, I have to pick up a hand stop and some sights.  I may also play with a longer rail section for the bottom for the hell of it.





B1gB1ue, per your request...

It appears the straight wall portion of the case is about flush with the barrel's breech face.

So, after 150-200rds and more jams than the Smucker's factory, I finally gave the ejector a slight tweak.  Now it spits empties clean and clear to the 3 o'clock position.

Nice 9mm AR Pistol and Shockwave Brace
So what was the ejector tweak?

Johnny

Thanks!
I just bent the ejector ever so slightly inward. There was slight gap between the ejector and the bolt.

Taking notes of the troubles and fixes.  :)  I'm hoping to take mine out this weekend for the first time.

One quick check is to pull the charging handle back until the ejector starts to poke out. There should be minimal gap between the ejector and bolt.

SLR has a new 4.75" rail out that would fit perfectly.

Of course they do...
Figures!