The HK51 is a variant of the G3 which is a select-fire battle rifle developed in the 1950’s by Heckler & Koch. 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.