I gonna start this because HK deserves it. I will put in more info as I have time.
There are 4 different distinct generations/types of MR223
MR223: 2007
Non-chromelined barrel
rear pin hole moved to be not compatible with standard AR15
Threaded but with no flash suppressor M15X1
Commercial dimension stock extension tube.
Standard AR15 trigger
MR223A1: 2009
Non-Chromelined barrel
BKA approved new lower/upper with standard AR15 push pin locations
Threaded but with no flash suppressor M15X1
HK 2 stage trigger with disconnector hood, can be put to safe with hammer uncocked
MR556(A1): 2010 - American version
Non-Chromelined barrel
Mil-spec stock extension
Standard AR15 push pin locations.
Lower with flared competition style magwell
Standard HK416 birdcage flash suppressor 1/2X28
Locked down push pins ( must use tool to unlock push pins)
HK 2 stage trigger with disconnector hood, can be put to safe with hammer uncocked
MR223A3: 2013 to present
based on HK416A5
Commercial stock extension
Chromelined barrel and chamber
Standard AR15 push pin locations
G3 type flash suppressor without locking ring M15X1
HK 2 stage trigger with disconnector hood, can be put to safe with hammer uncocked
New Slimlined buttstock
Difference between MR223 and MR223A3
Barrel and chamber: A3 is chrome lined
Barrel extension: starting at A3, HK no longer uses traditional AR15 index pin and slot arrangement. Not backward compatible with A1 upper
Lower: A3 lower has Ambi safety and bolt release
Upper: A3 upper has raised area to protect bolt release. Push rod bushing is roll pinned. A3 has more picatinny slots. Different barrel extension indexing slot from A1
Difference between MR223A3 and HK416A5
Barrel and chamber: different muzzle crowning and potentially slight difference in chambering
Barrel exterior: MR is approximately 0.15" thicker both in front and behind gas block
Gas block: MR uses A1 gas block. A5 has gas block with 2 settings
Barrel thread: M15X1 vs 1/2X28 tpi
MR has protrusion in barrel extension to prevent use of full auto BGC
Tools for "gun plumbing"
HK flash suppressor - 17mm wrench
Barrel Nut Wrench - "Giessele barrel nut wrench" for Giessele AR15 rails
Barrel Nut Torque - just above 32 ft lb ( this is not a factory spec, just by my own experience - use at your own peril )
Barrel Extension "reaction rod" - Magpul Biv block compatible.
FAQ
Q: Are MR223 and MR308 trigger groups interchangeable?
A: No, they look the same, but MR308 has different pin hole size to prevent installation of MR223 trigger group
Q: Can I remove trigger block safety from bolt carrier to use regular Giessele SSA trigger?
A: It will not be safe. HK has higher bolt velocity that may cause discharge by the forward inertia of the firing pin.
Q: Can I use aftermarket AR15 charging handle in a MR?
A: Generally yes.
Q: Does HK use standard AR15 sights?
A: HK used to have its own sight height for HK416 and MR223. Around 2014/15ish, HK began to supply BUIS in standard AR15 height with their MR rifles. In general, if you have a rotating "drum" rear sight, it is in the old HK proprietary standard. See picture below:
Q: Are MR556 and MR223A3 barrels interchangeable?
A: No. You cannot purchase a MR556 barrel from the US and install it in the Germany MR223A3. They have different barrel extension and indexing. See picture below:
Q: My gunsmith was cursing when he re-profiled a HK barrel. Internet said HK uses a tougher steel, so what is it really?
A: The exact steel used by HK is not known and they probably won't tell anyone. It was guessed by many that Aubert & Duval is a HK steel supplier because at one point A&D used a HK G36 in its "GKH" steel brochure.
There are at 3 melting modes of GKH and qualities of cleanliness. It is unknown which one HK actually uses.
Here is a comparison of what a typical US mil-spec steel to A&D GKH
Comparison of AISI 4140 and 4150 to the vacuum arc remelt mode of GKH. The Charpy V-Notch test measures the energy absorbed during fracturing. This partially explains why European military small arms passed barrel obstruction tests without rupturing, while some US made commercial barrels simply blows apart.
There are 4 different distinct generations/types of MR223
MR223: 2007
Non-chromelined barrel
rear pin hole moved to be not compatible with standard AR15
Threaded but with no flash suppressor M15X1
Commercial dimension stock extension tube.
Standard AR15 trigger
MR223A1: 2009
Non-Chromelined barrel
BKA approved new lower/upper with standard AR15 push pin locations
Threaded but with no flash suppressor M15X1
HK 2 stage trigger with disconnector hood, can be put to safe with hammer uncocked
MR556(A1): 2010 - American version
Non-Chromelined barrel
Mil-spec stock extension
Standard AR15 push pin locations.
Lower with flared competition style magwell
Standard HK416 birdcage flash suppressor 1/2X28
Locked down push pins ( must use tool to unlock push pins)
HK 2 stage trigger with disconnector hood, can be put to safe with hammer uncocked
MR223A3: 2013 to present
based on HK416A5
Commercial stock extension
Chromelined barrel and chamber
Standard AR15 push pin locations
G3 type flash suppressor without locking ring M15X1
HK 2 stage trigger with disconnector hood, can be put to safe with hammer uncocked
New Slimlined buttstock
Difference between MR223 and MR223A3
Barrel and chamber: A3 is chrome lined
Barrel extension: starting at A3, HK no longer uses traditional AR15 index pin and slot arrangement. Not backward compatible with A1 upper
Lower: A3 lower has Ambi safety and bolt release
Upper: A3 upper has raised area to protect bolt release. Push rod bushing is roll pinned. A3 has more picatinny slots. Different barrel extension indexing slot from A1
Difference between MR223A3 and HK416A5
Barrel and chamber: different muzzle crowning and potentially slight difference in chambering
Barrel exterior: MR is approximately 0.15" thicker both in front and behind gas block
Gas block: MR uses A1 gas block. A5 has gas block with 2 settings
Barrel thread: M15X1 vs 1/2X28 tpi
MR has protrusion in barrel extension to prevent use of full auto BGC
Tools for "gun plumbing"
HK flash suppressor - 17mm wrench
Barrel Nut Wrench - "Giessele barrel nut wrench" for Giessele AR15 rails
Barrel Nut Torque - just above 32 ft lb ( this is not a factory spec, just by my own experience - use at your own peril )
Barrel Extension "reaction rod" - Magpul Biv block compatible.
FAQ
Q: Are MR223 and MR308 trigger groups interchangeable?
A: No, they look the same, but MR308 has different pin hole size to prevent installation of MR223 trigger group
Q: Can I remove trigger block safety from bolt carrier to use regular Giessele SSA trigger?
A: It will not be safe. HK has higher bolt velocity that may cause discharge by the forward inertia of the firing pin.
Q: Can I use aftermarket AR15 charging handle in a MR?
A: Generally yes.
Q: Does HK use standard AR15 sights?
A: HK used to have its own sight height for HK416 and MR223. Around 2014/15ish, HK began to supply BUIS in standard AR15 height with their MR rifles. In general, if you have a rotating "drum" rear sight, it is in the old HK proprietary standard. See picture below:
Q: Are MR556 and MR223A3 barrels interchangeable?
A: No. You cannot purchase a MR556 barrel from the US and install it in the Germany MR223A3. They have different barrel extension and indexing. See picture below:
Q: My gunsmith was cursing when he re-profiled a HK barrel. Internet said HK uses a tougher steel, so what is it really?
A: The exact steel used by HK is not known and they probably won't tell anyone. It was guessed by many that Aubert & Duval is a HK steel supplier because at one point A&D used a HK G36 in its "GKH" steel brochure.
There are at 3 melting modes of GKH and qualities of cleanliness. It is unknown which one HK actually uses.
Here is a comparison of what a typical US mil-spec steel to A&D GKH
Comparison of AISI 4140 and 4150 to the vacuum arc remelt mode of GKH. The Charpy V-Notch test measures the energy absorbed during fracturing. This partially explains why European military small arms passed barrel obstruction tests without rupturing, while some US made commercial barrels simply blows apart.
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