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.