by Mel Carpenter

There are hundreds of different Gyrojet rockets ranging in size from 2.8mm to 55mm, but the only rockets made as pistol ammunition that actually went into mass production were those for the Model 137, Mark I, Mark I Model B, and Mark II Model C pistols. The other types and sizes were experimental and were not offered for sale to the military or civilian markets, or they were not for use as pistol ammunition. They are scarce. The examples covered here are the ones more likely to be encountered for sale to collectors. For those readers interested in details of Gyrojet rocket construction, including history, concept, advantages, disadvantages, components (igniter, case, nozzle, primer, propellant, and waterproofing seal), etc., please visit my website at www.gyrojet.net and see the free sample Chapter 6. Introduction to Gyrojets, which is available as a PDF download. .49 Caliber Rockets T 

49 Caliber rockets

The first production Gyrojet pistol was the Model 137. Its .49-caliber ammunition was first designed as a convenient way to make a rocket in the .50-caliber range, which MBA had determined through testing to be the best size for its application. Because money was in short supply, MBA co-founders, Robert Mainhardt and Arthur T. Biehl, Ph.D., visited local machine shops where they saw inexpensive half-inch steel bar stock, which had ample strength for use as rocket ammunition. Because the surface of the steel had minor flaws, the bar stock was turned down to “clean it up,” according to Mainhardt. This left a stock .49-inch in diameter, which is why the first rocket and pistol were .49-caliber. The first rounds were turned on a screw machine locally, which was a slow process. Once the initial design had been finalized, other cases were deep-drawn. Rockets, which were to be test-fired soon after manufacture, were left plain to save money. Others, which would be packaged and distributed away from the  MBA, were copper-plated to prevent corrosion. Dummies were made to check pistol function and as souvenirs. Nickel-plated dummies were used in cased presentation sets. Five .49-caliber.

13MM rockets 

The most “common” Gyrojet rocket is the 13mm version because it was made in the largest quantity, by far. There are more variations of this round than any other Gyrojet. The .49-caliber rocket proved to be unsatisfactory because it was weak and inaccurate. To solve the case weakness, the rocket’s outside diameter was increased to 0.51 inch (but no increase to its inside diameter), resulting in a thicker, stronger case. The new rocket was designated 13mm, in part to separate it from the old version. Most 13mm rockets have a round nose ogive similar to the .45 ACP cartridge, with better ballistics, but others were made as wadcutters, with straight cases and conical noses. Others used hollow, two-layer, rolled, and brazed “Bundy tube” as case material. These have an aluminum rivet in their noses to close the hole left when the nose was formed from the hollow tube. There is a wide range of 13mm dummies, and the most common are the bright nickel-plated versions seen in cased presentation sets. One very special tin or zinc-plated dummy was autographed by Mainhardt for me during one of our interviews

 

12MM rockets 

Because of the Gun Control Act of 1968, firearms with bores larger than 0.50 inch were deemed to be destructive devices, with a few exceptions based on suitability as sporting weapons. Therefore, the caliber of Gyrojet pistols and ammunition was reduced to 12mm (.49 caliber). Some, if not most, existing 13mm rocket cases were simply resized to 12mm, just a 0.4mm reduction (13mm cases actually measure about 12.94mm, and 12mm cases measure 12.54mm). At this time, MBA was developing a new nozzle made from powdered metal, and almost all 12mm rockets used that nozzle design.

 

Gyrojet Cartridge Boxes 

Gyrojet rocket ammunition was generally packed in 24-round (four loads of six rounds) off-white cardboard boxes, but other cardboard and plastic boxes were tried. Because the ammunition was so expensive, MBA offered 12mm ammunition in 6-round boxes in addition to the 24-round versions.