And here’s the carbine version of the Gyrojet rocket gun (previously shown in pistol form). It’s one of those great ideas that really just didn’t work all that well. I still think a modernized version could be made into a practical and entertaining weapon, but as pointed out in the video the original Gyrojet carbine, if made today, would instantly get you in trouble with the feds. The rounds were 13 millimeters in diameter… 0.3 mm greater than anti-firearms laws passed in 1968 would allow (anything above 50 caliber is considered a “destructive device, except, somehow, shotguns).
A modernized Gyrojet in carbine form could probably be made to burn propellant fast enough to burn out just before the end of the barrel, so that it would have full velocity right at the muzzle. Or at least have a boost/sustain rocket grain to get a sizable fraction of the energy at the muzzle. And give it a decent replaceable magazine, for frak’s sake. And a solid-fuel ramjet sustainer…