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.
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.)
The bolt carrier group has a spring loaded firing pin:
And a firing pin safety:
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:
The barrel extension is longer on the HKs preventing interchangeability with AR15 barrels.
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.