THE HECKLER & KOCH ROLLER SYSTEM IN THE MP5
Of all of the weapons to be found in the hands of counterterrorist forces, elite military forces, and law enforcement, by far the most common has been the Heckler and Koch MP5 submachinegun and its variants. The weapon evolved from the successful HK family of rifles, primarily the G3 and variations, as used by the West German military as well as military forces around the world. The MP5 was the natural evolution of the G3 rifles, adapting the system to use a 9x19mm cartridge rather than a full-sized rifle cartridge (7.61x51mm).
In the closing months of World War II, the German military was developing a new version of their assault rifle. This new rifle would be the Stg 45, with the Mauser-designed Gerat 06H being the most likely contender. The new rifle would use rollers to retard the bolt from opening. This was not going to be a true locked bolt but what the Germans called a “Half-locked” action, hence the final “H” in the designation. The system worked and was relatively simple once the engineering was worked out. The new weapon was in the prototype stages when the war ended and all German weapons production ceased.
After the war and the fall of Nazi Germany, the designers of the Gerat 06H went to the CETME arms works in Spain where they perfected their design for the Spanish government. The Stg 45(m) would have utilized much the same roller locking system of the battle proven MG42 light machine gun, but was chambered for the 7.92x33mm Kurz (Short) assault rifle cartridge. The first of the new weapons produced by CETME was chambered for a special 7.9x40mm cartridge firing an exotic bullet but the cartridge was never widely accepted and soon died out. The basic CETME design was further modified and perfected for use with the new NATO 7.62x51mm round and was submitted for trials with the new West German army. The rechambering and redesign of the CETME rifle was accomplished in Germany by the new Heckler and Koch company as their first major weapons development. The new rifle passed all tests and was adopted by Germany as the G3 in January, 1959.
The roller-locking system does not actually lock up the breech of the weapon when it is fired. In effect it is a form of delayed blowback that greatly simplifies the design of the weapon in that it eliminates the need for a gas system to operate the action. But it carries with it some substantial problems. The lack of a rotating bolt limits the primary extraction of the cartridge case as it is being fired and is under high internal pressure. The fired cases adhering to the chamber walls and sticking pose a real difficulty. The sticking case problem is solved in the H&K system by the cutting of a series of longitudinal flutes along roughly two-thirds of the length of the chamber. The flutes allow the hot propellant gases to flow along the body of the cartridge case along the area of its greatest expansion, “floating” the casing on a layer of gas and easing extraction. The fluted chamber/gas floatation
system eliminates much of the sticking casing problem, but does give the roller locking system a very violent case ejection. The ejected case often are dented from striking the ejection port and also have a series of distinguishing marks on them that identify the weapon they were fired from.
The complete roller locking system works via two rollers in the bolt that are driven into locking recesses (angled locking flats) in the barrel extension by the forward movement of the wedge-shaped control piece that carries the firing pin. Rearward pressure of a cartridge being fired bears against the bolt head and drives the rollers against the rear of the recesses in the barrel extension. The angled rear surfaces of the barrel extension recesses force the rollers inward against the angled surface of the control piece, driving the piece backwards but at a great mechanical disadvantage to the force of the rollers. This mechanical disadvantage the rollers work against slows the opening of the breech until the bullet has left the barrel and pressure have dropped to safe levels.
