The Studies and Observations Group (SOG) of the U.S. Military in Vietnam in 1967 saw an opportunity to try out one of SOG's new "toys," a revolutionary rocket pistol called a Gyrojet. In one test, a rocket round punched through an old truck door and into a water-filled 55-gallon drum, almost exiting its opposite side. SOG men also test-fired it through sandbag walls and even tree trunks.
Yep. Last check on the status of supply and market value of the Gyrojets and all it's accoutrements, the civvie-legal Mark II .49 caliber rockets run around several hundred dollars per round. The carbines, and ESPECIALLY the assault rifle variant with the carrying handle will run up to $60,000. A lot more Mark II model pistols were made. These were built for sale on the commercial market while the few .51 caliber Mark I's were submitted as test prototypes to the US Army. There was also the even more rare Lancejet, a flechette rifle.
Here is something interesting: To promote the sales of the Mark II on the commercial market, MBAssociates even tried to persuade Gene Roddenberry to equip the cast of Star Trek, which was just making it's cameo appearance on TV, with Gyrojet handguns. But upon handling and shooting a few rounds out of one, Roddenberry wholly rejected the idea. He wanted to portray a total 'futuristic' setting so only energy weapons would be used by the Starfleet crew. After all, rocket pistols are still solid missile projectors like their mainstream brethren.