It’s a homemade imitation of the Swedish-made Carl Gustav recoilless rifle, which was used primarily in the 1950s and ’60s
The author obviously confused the Carl Gustav recoilless rifle with the Carl Gustav M/45 9mm submachine gun. The recoilless rifle is about 41 inches long, fires a single 84mm projectile, and weighs about 20 lbs. It has an excellent reputation, but it is not so easily reproduced in small shops, and the ammunition is far harder to come by.
The author is most certainly not a veteran of the 1956, '67, or '73 wars, nor familiar with the 1981 assassination of Egyptian dictator Anwar Sadat, or he'd have known the difference between a Charlie-Gee antitank weapon and the Port Said Egyptian copy of the Swedish M45b kulsprutepistol SMG- which was used to assassinate Sadat:
