This video shows an Hellenic Navy Rheinmetall Mk20 with S20 Naval Gun Mount firing in slow motion. Note how the barrel appears to recoil. At first glance it appears that the gun utilizes a short recoil action like the Rheinmetall MG3 and the MG42. It is actually gas operated but the whole barreled action is mounted on a recoiling platform in order to soften the recoil.
[ Many thanks to Aegeanhawk for the video. ]