The 51 is not a Heckler & Koch creation, it started life in the gun shop of Bill Fleming owner of Fleming Firearms. In simple terms the HK 51 is basically a HK 91, or any of the clones that is cut down to the size of a HK MP5. The 51 uses the same forearms as the HK MP5. The rear end of the 51 remains the same so only 91 stocks will work. The 51 has a 8.?? inch barrel and the roar of a 50 cal. This is considerd a short rifle and need a special license to own.
The 51S(S = Semi-Auto) is the same as the above but needs no special license. It has a 16.25 inch barrel and the basic S-F trigger(Semi-Auto).