A look inside the HK MR556 (HK416)


This week I saw someone posting online that the military prefers the HK416 because it is simpler than the M4.  I had a really hard time not signing up for that forum to call the poster ignorant.  Here are a few pictures showing some of the differences from an AR15 to an HK.

The HK416 and the MR556 use a short stroke gas piston system.

2016-05-04 18.31.38

The piston sits in the gas block.  Note that the one below has a broken gas ring.  ($20 for 3 replacements from HKParts.net, compared to $3 to replace an AR15s gas rings from Bravo Company.)

2016-05-04 18.34.56

The bolt carrier group has a spring loaded firing pin:

2016-05-04 18.33.31

And a firing pin safety:

2016-05-06 20.18.09

Note that because of this firing pin safety, HK uppers won’t work with many various hammers.  For example, my Geissele SSA is unable to fire a HK upper.

The upper it self has an insert for the spring loaded piston rod:

2016-05-04 18.32.13

The barrel extension is longer on the HKs preventing interchangeability with AR15 barrels.

2016-05-04 18.34.38

The top barrel is a MR556 barrel.  Note the protrusion to prevent being used with a full auto bolt carrier group.  The bottom barrel is an aftermarket replacement.


  1. Yup. The HK416 military model is well engineered, it’s one of the best made. The truth is that piston systems are in many way inferior to direct gas impingement, in that they add weight and complexity in moving parts. The mid-term maintenance is a “nightmare”, as quoted by folks who use them professionally. I have a hard time taking feedback without having experience with it myself but for now I’m not interested in investing 3K into an HK piston model. I’ll stick with a customized DGI Stoner for the time being.


Please enter your comment!
Please enter your name here