Matt with The Armorer’s Bench spends some quality one-on-one time going over the HK53, a shorty 5.56mm carbine reduced roughly to the same size of an MP5.
The Heckler & Koch HK53 was designed in the 1970s as an ultra-compact version of their 5.56mm HK33– basically the West German answer to the Warsaw Pact’s AKS-74U Krinkov or Colt’s various Vietnam-era Commando and XM177 models. They saw some minor export success, and in the U.S. the pre-Homeland Security Border Patrol adopted them for some tactical teams back in the day (hey, Customs had the Austrian-made Steyr-AUG at the same time, so you can see the need for competition).
Today they are just available as clones from Dakota Tactical and others as the parent company moved on to the G36 series some time ago.
Then again there is also the same concept in 7.62x51mm NATO, the near-mythical HK51.