Welcome to today’s Photo of the Day! Here we have 48 rounds of Gyrojet 13mm rockets from someone whose great grandfather worked for MBA (MBAssociates) in California during the late 1960s and early 1970s. This ammunition turned conventional firearms ballistics upside down.
Gyrojet rounds weren’t traditional cartridges – they were miniature rockets. Instead of powder pushing a bullet down a barrel, the Gyrojet projectile had four canted exhaust ports at its base that produced thrust. When fired, these ports vented gases that both propelled and spun the projectile, creating gyroscopic stability. The rocket motor burned during flight, meaning velocity actually increased as the round traveled downrange.
Robert Mainhardt and Art Biehl founded MBAssociates in San Ramon, California and developed the Gyrojet system in the early 1960s. The concept offered virtually no recoil since there was no explosive pressure inside the gun, minimal barrel wear, and ammunition that got faster at distance rather than losing energy. The 13mm rockets used solid fuel propellant inside the projectile body. A conventional primer in the base ignited this fuel when struck by the firing pin. As fuel burned, exhaust gases escaped through four angled ports, creating forward thrust and rotational spin.
The fatal flaw was muzzle velocity. Traditional firearms achieve maximum velocity instantly. Gyrojet rounds left the gun at around 100 fps, then accelerated to roughly 1,250 fps at 60 feet. This meant Gyrojets were nearly useless at close range where defensive shooting happens.
MBAssociates produced ammunition in 13mm and 12mm sizes for pistols, carbines, and prototype underwater weapons. Loadings included armor-piercing, explosive, and flare rounds. Production ran from the mid-1960s through early 1970s before financial collapse. Finding original Gyrojet ammunition today is difficult since relatively small quantities were manufactured.
Most of our POTDs utilize images from our friends at Rock Island Auction Company, the premier firearms auction in the United States. Take some time to browse their current auctions – who knows, maybe you’ll find a piece of history to take home!
“Gyrojet 13 mm Rocket Ammunition.” Rock Island Auction, www.rockislandauction.com/detail/1049/3798/gyrojet-13-mm-rocket-ammunition. Accessed 1 Feb. 2026.
The post POTD: Gyrojet 13mm Rockets – The Ammunition That Was Its Own Engine appeared first on AllOutdoor.com.
