The Helenius RK-97 APH is a heavy caliber anti-materiel rifle originating from Finland. The system is available in two distinct forms to make use of the NATO 12.7x99mm NATO or the Soviet 12.7x107mm cartridge as needed and is a single-shot, breech-loaded weapon. The use of these two popular heavy-caliber rounds ensures interest on the global market for such a system. Externally, both versions of the weapon appear similar and both make use of a "bullpup" style configuration - that is, the magazine feed and action are unconventionally set behind the trigger group.
The stock is constructed of light alloy which has a padded buttplate and synthetic cheekpiece. The barrel measures 44 inches long and is made from heavy grade steel - it is specially designed to deflate the effects of the massive recoil inherent in firing such a large-caliber cartridge. The muzzle brake is removable and sports a "pepperdot" pattern. A bipod supports the forward portion of the weapon and is non-adjustable but foldable to varying degrees, in essence becoming "indirectly" adjustable. Amazingly, this heavy and large caliber weapon has been successfully tested with sound suppression equipment - a suppressor simply threaded onto the barrel in place of the muzzle brake. As no iron sights are featured, optical sights are required for effective operation. The optics are mounted on ring-type fixtures along the left or right side of the receiver and can be of various makes as dictated by the operator. Attachments for iron sights are present however, should the operator need to make use of a standby sighting apparatus.
The operator engages the weapon by pushing on the foregrip which, in turn, drops the breech block (the RK-97 is categorized as a "breech loading" weapon system). The breech block itself is unique as it is a "vertical dropping" breech block system. A fresh cartridge is entered into the breech by hand to which the operator then returns the foregrip to its original position, locking the breech block in place. The weapon is now made ready to fire. In essence, the RK97 utilizes an action akin to a traditional shotgun's "pump-action" method.
The RK-20 APH is a larger version based on the RK-97, chambered to fire the World War 2-era Russian 20x99mm ShVAK cartridge (the Finns designate this cartridge as the 20x100mm Swak). The RK-20 sports a shorter (33.9 inches) but larger overall barrel assembly with applicably-sized muzzle brake and a revised internal action for the larger cartridge.
The RK-99 APH was introduced in 2000 an updated form of the original RK-97 and utilizes a detachable box magazine feed for a repeat firing via a rotating bolt internal mechanism. The barrel is 32.4 inches long and its internal functions are naturally different from the base single-shot RK-97. The RK-99 is available with a 5- or 10-round detachable box magazine. Though the 10-round shot capability increases the battlefield staying power of the RK-99, the added height of the magazine works against the RK-99's bullpup configuration in some ways.
The RK-99MK1 is another single-shot form of the rifle with a different firing action that improves reliability and promotes simplified use of varying ammunition types. The RK-99MK2 is chambered for the 20mm ShVAK cartridge and features a shorter 31.5-inch barrel length.