In the early sixties, Castle State Arsenal was charged with modifying the AR-18 to meet the standards of the Cordian Military. These efforts initially produced the ART-64, which was found satisfactory by the military, but Castle developers, including Arnie Castor and Wilbur O’Kanna, felt that the weapon’s true potential had not yet been met. Castor and O’Kanna worked together to continue where the ART-64 project had left off, with the intent of producing a weapon that could be easily and cheaply manufactured almost anywhere, and which could be made to serve in almost any role. The MRS-67 emerged as a cheap, reliable and accurate platform that could be easily produced, repaired and modified to fit practically any need. The weapon never saw formal adoption by the Cordian military, but Cordian Expeditionary Rangers and SIAS Commandoes swore by it, and the weapon was used extensively to equip nations in exchange for aligning with NACO and opposing the Kursk Accords. Various local versions are still produced today, and Castle offers a modernized version for sale both to governments and civilians.
The first prototypes of the MRS-67, the XM-65 and XM-65A, were basically attempts to improve the ART-64’s multi-purpose functionality. The XM-65 added a integrated folding ladder sight and spigot for launching rifle grenades, and the XM-65A cut open the top of the receiver, and added a milled steel guide for 20 round strip-style clips. However, it was quickly realized that the opened receiver aggravated already-present issues with the ART-64’s reliability in sandy environments, and a new bolt was designed to help alleviate the issue. The new bolt used a single, stiffer return spring instead of the ART-64’s two smaller ones, and had an integrated clip guide milled into the top, as well as slightly looser tolerances to accommodate dirt and sand particles without impairing the bolt’s functioning. In order to deal with the new bolt, a new upper receiver had to be designed. This weapon was called the XM-66 During this process, Wilbur O’Kanna had the idea to allow the rifle to accommodate larger, “full-power” rounds as well. The receiver was lengthened and re-enforced, as was the bolt, and they decided to go back to two springs, but using the larger, stiffer springs of the XM-66 instead of the original ART-64 springs. Wilbur wanted to use the same lower receiver for both calibers, but Arnie Castor had a different idea; he worried that the extra magwell length would cause more foreign material to enter when the weapon was being used as an assault rifle, and he proposed using a separate lower for the two calibers, while retaining the same upper. This also eliminated the possibility of full-power rounds being mistakenly loaded into a weapon with an intermediate barrel and bolt. It also lead to a complete re-design of the lower, to facilitate easier swapping in the field.
By this point, the weapon bore little resemblance to its parent, except in gas system and barrel design. And even that was about to change with Wilbur’s next idea. Already, the weapon had the potential to replace most of the weapons currently fielded, save for the machine-gun. Failing to see a reason why he should stop there, he designed a replacement upper receiver which could be fed with the same belts as the newly adopted MGT-66 family. This required a redesign of the piston system as well, and by the time the final prototype, entitled XM-67E, was built and tested, all vestiges of the ART-64 were gone. The XM-67E met with enthusiasm with Castle executives, and the weapon was added to the product line as the Modular Rifle System, model of 67, or MRS-67.
The MSR-67 utilizes a conventional layout and a gas-operated, rotating bolt short-stroke action. The receiver comes in two parts; upper and lower, which can be separated and swapped out at will to allow the weapon to perform various tasks. The upper receiver remains the same for every configuration except LMG, while the lower receiver, barrel and internal parts dictate the caliber. The upper receiver contains the sights, return spring guide rods, bolt carrier assembly, ejection port, cocking handle and receiver trunnion. The LMG upper receiver features a hinged receiver cover with belt feed pauls, a feed guide and steel flange on the left side for mounting a belt drum, and a downward-ejecting bolt. The LMG upper contains a pair of tangs which disable and enable the closed bolt and open bolt sears, respectively, locking the weapon in open-bolt mode. The lower receiver contains the magazine well, pistol grip, fire control group with closed and open bolt sears, and stock trunnion. Receivers are held together with takedown pins, which can be removed in the field with a pin punch, pencil, utility knife or other slim, pointed object.
The barrel and gas piston attach to the upper receiver via a milled trunnion, and are sheathed with a two-piece handguard made of anything from bakelite to metal to wood, depending on the model year and who is manufacturing the weapon. The gas block is a heavy, milled design with a built-in regulator valve, and a folding ladder sight along the top which can be folded forward to allow use with rifle grenades. When folded into the upright position, the ladder sight seals the gas outlet, turning the weapon into a straight-pull bolt action rifle. The gas block in most models has a swiveled sling attachment ring attached to the right side, and a hooded front blade sight located above the gas knob and in front of the ladder sight. The barrel, which can be provided in various lengths and thicknesses, is caliber-dependent, and secured to the receiver trunnion with a milled nut. It is threaded at one end for attaching either a rifle grenade spigot, or a NACO standard muzzle device. The bolt head has five lugs in the intermediate chamberings, six in the full-power, and a built in extractor and ejector, and bears some similarity to an AR-15 type bolthead, though with looser tolerances. Each caliber has it’s own size and shape of lug, unique to it, though some third-world copies and reproductions may not remain true to established dimensions. The difference in boltheads between calibers prevents the weapon from locking if caliber parts are mismatched, and avoids a potentially disastrous malfunction. Though it is not recommended to count on this, and users are advised to personally verify the consistency of parts before firing an MSR-67 or derivative thereof.
The bolt carrier itself has a built-in guide for loading the magazine via NACO pattern strip-style clips, is rectangular in profile, and rides on a pair of stiff recoil springs and spring guide rods. The MSR-67’s bolt carrier makes use of the “constant recoil prinicple” demonstrated in Cordian MGP-1948 heavy machine-guns, never actually touching the rear of the receiver, and shedding all it’s force into the springs. This keeps the rate of fire low, and makes the weapon slightly heavy to cock, but reduces recoil and allows remarkable controllability during automatic fire with intermediate cartridges. Auto fire with full power rounds is discouraged, and some full-power lower receivers lack a full-auto sear, fixing the weapon in semi-automatic mode.
The stock varies greatly between variants, model years and manufacturers, but all types use roughly the same stock trunnion located at the back of the lower receiver. Some models have been made to accept Kalashnikov stocks, while other, more modern versions use an AR-15 style telescoping stock. The sights are adjustable for windage and elevation, and consist of a peep-style rear and a blade-style fore. Some are painted with tritium or a similar luminescent material to allow use at night. Rifles made after 1989 often incorporate a PCAP strip along the top of the receiver for mounting optics, and some use picatinny rails. The weapon usually feeds from NACO STANAG magazines, either of the intermediate or full-power variety, but the modular construction and split receiver design allows the use of other types, including AK magazines.
The MRS-67 is primarily made of stamped steel, with the bolt carrier assembly, fire control group, gas block assembly, gas piston, various trunnions and the sights being milled. This gives the weapon fairly loose tolerances and a flexible, durable nature that makes it extremely tough and reliable. It is reported to be almost as reliable as an AK-based weapon, but stress tests to that effect have not been conclusive. Furniture varies wildly by producer; some use wood, others bakelite and still others stamped steel. Most Cordian-produced weapons use the familiar near-indestructible green polymer-coated fiberglass furniture common to Cordian weapons of the era, but versions have cropped up with everything from AR-15 style telescoping stocks and quad rails to wooden handguards and pistol grips with MP-40 style underfolding stocks.