Cartridge with a Polish flare gun from the 1950s based on the Soviet SPSh-44.
Cartridge disassembled, most parts are machined from 12L14 steel apart from the body that is 6061 aluminum
Operating principle roughly to scale. The effective area of the piston is just 0.157 square centimeters or 0.024 square inches, but at 3000 psi that's a force of 72 lbs on the ball locks, in hindsight that difference in diameter could have been smaller.
Cartridge inserted in the launcher
Video with a more detailed explanation of the mechanism and some action shots
Test slugs made from 12 g CO2 capsules, the top two have small screws inserted in the nose and lead filling on the inside.
From top to bottom here are the weights and measured velocities and muzzle energy:
19.8g - 277 fps - 52 ft lbs
13.0g - 401 fps - 72 ft lbs
7.9g - 486 fps - 64 ft lbs
Normally more projectile weight gives more energy, but a dynamic at play here is that the projectile tail being hollow means there is more "dead" volume in the barrel, giving the air more space to expand and therefore dropping pressure.
Aluminum dart with a steel core, weight is 20.7 grams but I didn't measure the velocity, presumably it's below 300 fps.
