Basically there are 3 solutions that do not include messing with barrel on HK MR223:
- Get muzzle device with M15x1 thread straight from manufacturer.
- Re-thread muzzle device from 1/2x28tpi to M15x1
- Make screw on adapter that is female M15x1 thread and male 1/2x28tpi thread.
Ad 1. There are some manufacturers who make or can make AR15 style muzzle devices with M15x1 thread. Good example is Arredondo Accessories, who can deliver their (very good by the way) nXg brake/comp with internal M15x1 adapter (they can also do other threads, like M14x1 for Oberland Arms rifles). Other choice is Primary Weapon Systems brake/comp FSC556 HK (or FSC91 that is universal fit on HK MR223 and MR308, as well as G3). There is also number of European companies who make their own M15x1 muzzle devices or just make versions "inspired" by products of US companies.
Ad.2 Several types of muzzle devices can be modified by skilled gunsmith or machinist from female 1/2"28tpi to M15x1. Important part is that muzzle device must have enough material around thread portion to allow enlarging threads. For this reason any muzzle device that mimics A2 flash hider in this regard can not be modified. This applies for example to Battlecomp brake/comps or several other similar designs. I had success with rethreading JP/Cooley Tactical brake/comp. Also I sold DPMS Miculek Brake to friend who re-threaded it for MR223 with success. Also devices like Ares Armour EFFIN-A (very nice comp by the way) can be modified this way. I did not check it, but I think (no warranty whatsoever) that comps like SJC Titan, Benny Hill Rolling Thunder or Dreadnaught Industries F2 can be modified this way.
Ad 3. Most universal solution is to have gunsmith (or good machinist) make adapter unit (this is what I currently use). This way one time cost allow to try different muzzle devices and find one that works best for personal needs. There are of course downsides, like added weight. Also added space inside male threads (some 15mm long tube) works as mini expansion chamber, that can reduce effectiveness of high pressure type muzzle brakes (like SJC Titan). But devices like GoGun USA Supercomp or Ares Armour EFFIN-A seem to work very well. I also used this approach to mount AR15 muzzle devices on my Mini Beryl short barrel rifle (picture), screwing on adapter in place of original 'flash hider/grenade adapter'.
What is most recommended solution? Of course solution 1, but it offers very narrow choice. Second best is solution 3, especially if shooter want to try/test several types of muzzle devices. However when one muzzle device is selected and it can be modified, solution 2 beats solution 3 in simplicity, lower weight plus allows device to work as intended, without additional "chamber".
Photo: My HK MR223 with Ares Armour EFFIN-A mounted on thread adapter laying next to HK416A5 prototype.