The CETME, and by extension the G3, had never been designed to fire a powerful round like 7.62x51mm NATO. CETME and Heckler & Koch had worked hard to reinforce the rifle’s receiver and introduce the use of high carbon steel.
Rheinmetall, who along with H&K, had won the contract to manufacture the G3 for the Bundeswehr developed a variant of the G3 chambered in the Soviet 7.62x39mm intermediate cartridge. The Rh4 used a long-stroke gas piston system believed to be designed by Henk Visser, however, contemporary patents suggest Theodor Loffler worked on on the gas system. Instead of using Ludwig Vorgrimler’s roller-delayed blowback action, the Rh4 fired from a locked breech with the rollers only unlocking once the projectile had passed the gas vent.
Two prototype Rh4′s are believed to have been manufactured. They fed from 30-round AK-pattern magazines and had a distinct hand guard which attached to the receiver by a pin just in front of the magazine housing. Rheinmetall struggled to get the G3 into full production and H&K took over prodction later developing their own intermediate cartridge versions of the G3: the HK32 chambered in 7.62x39mm, which never went into production, and the more successful 5.56×45mm HK33 family.