Gyrojets
The Gyrojets were a family of unique firearms developed in the 1960s. Firing small rockets rather than inert bullets, they had little recoil and didn't require a heavy barrel to resist the pressure of the combustion gasses. The result was a very lightweight weapon with excellent ballistics, and today they are a coveted collector's item.
Robert Mainhardt and Art Biehl joined forces to form MBAssociates, or MBA, in order to develop Biehl's armor-piercing rocket rounds.
Gyrojets
Originally developed in a 13 mm caliber, the cartridges were
self-contained self-propelled rockets.
Spin stabilization was provided by angling the four tiny rocket
ports rather than by forcing the projectile through a rifled barrel.
Combustion gasses released within the barrel were vented through the
vent holes in the barrel. Due to the properties of the rocket, with
continued acceleration after it left the barrel, the Gyrojet had
poor short-range power, but improved dramatically with distance.
After about 75 yards (70 m) the motor would burn out, at which point
the pistol-sized round had about 50% more power than the common Colt
.45.
A family of weapons was designed, including a pistol, carbine,
"rifle" and squad-level light machine gun. However only the pistol
and carbine were built. The space age-looking carbine versions were
tested by the Army, where they proved to have problems. One issue
was that the vent ports allowed the humid air into fuel, where it
made the combustion considerably less reliable. The ports themselves
could also become fouled fairly easily, although it was suggested
that this could be solved by sealing the magazines or ports.
Reports of the gun in testing showed that the accuracy was very
poor. However this contrasted with earlier reports that showed it to
be more accurate than conventional arms. It appears the difference
was due to the quality of the ammunition, and a small design flaw
that potentially blocked one of the ports.
The Rocketeer pistol was produced to the tune of about 1,000 copies,
and was featured in a James Bond book and movie. At about the same
general size as the Colt .45, the Gyrojet was considerably lighter
at only 22 ounces (625 g). The weapon was cocked by sliding a lever
above the trigger forward to pull a round into the gun, which sprung
back when the trigger was pulled. The lever hit the bullet on the
nose, driving it into the firing pin. As the round left the chamber
it pulled the lever forward again to recock to fire again. One poor
bit of design was the magazine: rounds had to be pushed down from
the open "bolt" and then held in place by quickly sliding a cover
over them on the top of the gun. Reloading quickly was impossible.
It appears there is nothing inherently wrong with the concept, other
than inadequate initial firing velocity. A round can actually be
stopped if a finger or even piece of cardboard is held directly at
the end of the barrel. This flaw is fatal, however, for a firearm
intended to be used at relatively close range.
As previously mentioned any rocket velocity was a product of thrust,
and time (i.e. distance). At close ranges this led to blocked
rockets simply stopping and burning out relatively harmlessly,
making it useless for close-range self-defence. On the other hand
since velocity actually increased over some distance the projectiles
trajectory did not suffer as great a ballistic drop compared to
conventional ammunition, greatly simplifying aim at longer ranges,
an advantage for offensive military uses.
The Gyrojet principle was also examined for use in survival flare
guns, and a similar idea was explored for a grenade launcher.
Some fiction like the Cyberpunk 2020 supplement Deep Space consider
Gyrojets as weapons of choice in space, thanks to its lack of
recoil.
Another interesting note is that in the science fiction continuity
of Warhammer 40,000 an almost identical concept is used (bolter
weapons), despite being almost 38,000 years into man's future.