The HK416c is an ultra-compact variant of the Hk416 originally developed for a UKSF tender. The HK416C has a 9 in barrel and is expected to produce muzzle velocities of approximately 2,395 ft/s. Though the HK416C has a high degree of component commonality with the HK416 family it has a HK416C specific shortened buffer tube and a sliding collapsible butt-stock similar to the ones used on compact variants of H&K's MP5 sub-machine gun.
Quoting Darkbird 4
Not big fan of the gun in reality, but your lego Representation is fantastic! The receiver is awesome. I don't know how you build your mags, can you spill the beans?
Thanks, now I have to figure out what to make next. Anyway, for the mag curve: stack 8x2 bricks with 2x1 plates in between and keep it together with black electrical tape. Simple (in my opinion), yet effective.