The HK51 is a variant of the G3 which is a select-fire battle rifle developed in the 1950’s by Heckler & Koch. The G3 had many variants including law enforcement and civilian models, including the HK51.
The HK51 in particular is not made by H&K, and is rather a creation of the American Class II manufacturing after-market. While the HK51 does not have any real standards, it is usually a modified G3A3, or the semi automatic HK41 and HK91, with MP5 furniture and accessories as well as a collapsible stock. With an 8.3 inches long barrel, the HK51 is rather small at only 30.7 inches long with the stock extended.