My AR-9 Pistol Build

[halo2304]

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.

[halo2304]

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.

[halo2304]

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.

[U&A]

Cut a rail that you like. that is exactly what I would do. I think it would work best in a band saw that has a vice unless you have a mill. cut, de-burr, sand the edges and paint.

[halo2304]

Here it is, almost finished.

[misfit47]

Looking good. Should be more fun to shoot.

[halo2304]

Looking good. Should be more fun to shoot.

Thanks, but here's an update. It absolutely is fun to shoot.

[U&A]

Thanks, but here's an update. It absolutely is fun to shoot.

what did you end up doing with your rail dilemma? did you cut one?

[halo2304]

I had a Midwest Ind. KeyMod rail cut down to 4.7ish inches. It came out really nice.
Recently, I learned SLR Rifleworks just came out with a short rail about the same length.

Mine's better...at least, that's what I'm telling myself.