A big variety of new guns. All source images are IMFDB.
HK21E modified with the entire front half and barrel of an M249 Para (stretched out to mimic the HK21E's length) and a Mk. 48 Mod 1 stock.
A very simple job: M240D with the barrel, front sight, and gas tube from a PKM shortened and widened to resemble a compact .50 caliber weapon.
A more complex job. Base is an StG-57 with the magazine from an M14, an MP5A2 stock, an FAL handguard with the vent holes removed to resemble indentations, an M14 flash hider, and optionally the scope off a Steyr Scout image.