The Guns Of The Future show up in many settings, but are usually one of three categories; lasers, gyrocs, or gauss guns. Guass guns are someone else's problem, and lasguns are fantastic and I have some musings that will require I brush up on electrical engineering a bit to start assembling a thing. This discussion is about TINY ROCKETS. A favored weapon in WH40k, along with lasers, which is amusing because the setting always drastically oversells the difficulty and complexity of a gyroc weapons system,
Of the three classes, gyrocs are ahead of the curve in terms of realism; the Gyrojet Pistol (sort of) worked, in the 1960s! Rocket motors have improved substantially in power since then, much like all other things with propellants, and the means to design with. With ammunition that didn't cost an arm and a leg ($7.50 per round or so to purchase at time of introduction, according to GURPS High Tech, so there might be a bit of error in there due to conversion factors to GURPS Dollars), combined with a gun that wasn't stupid, it might be darn viable.
First up, things wrong with the Gyrojet pistol; designed to be recoilless. That's all well and good, but if it makes your gun essentially nonfunctional out to 20 feet but is designed to be used as a defensive weapon? All the power in the world "eventually" doesn't help much if it does nothing "now" which is where you need it. Just sealing the barrel and permitting the rocket exhaust to pressurize it like a standard gun would probably step the power up substantially at the muzzle, as would using a relatively weak firing charge to ignite the round and get it up to speed while the rocket motors were still kicking in. Boosting muzzle velocity also helps accuracy, because the round leaving the gun at no speed means wind plays hell with your accuracy. Modern propellants may permit the gun to remain recoilless or semi-recoilless, if they can accelerate to a useful degree even at short ranges.
Several other issues with the most commonly known version of Gyrojet were solved by later models, when it was realized that designing things to be New and Different for its own sake was stupid, because a lot of how things worked was because that's what WORKED. As such, I won't discuss those particular changes and will merely state that a new production gyroc must have the features and capabilities one would expect of a modern firearm (detachable magazine, ability to clear jams should they occur, etc).
Next up, the ammo needs to be redesigned. Gyrojet rounds use angled vents to spin the round as it flies. Any problems in manufacturing those vents and the round turns into a crazy rocket, which isn't very useful. They also require fairly good tolerances be held, and it's not a particularly easy design to make conveniently. Instead, switch to a 2-3 part construction: the warhead is simply a bullet of the same caliber as the gyroc that press/crimp fits into the gyroc-to-be, the body of the round is simple tube metal that is formed to produce the rocket vent and fit to the final piece, a stamped metal fin. Stamp fin, roll-form main body, press/weld fin and body together, insert rocket motor, insert warhead. Gotta jam a primer and possibly the booster charge in there at some stage of that, but it wouldn't be at all hard to automate the heck out of that like any other modern ammo production system, to achieve competitive prices to same. Alternatively, using a sealed barrel and/or booster charge, you could use rifling to achieve the stabilizing spin with the rocket solely focused to driving the projectile.
The final question; what's the point? Does a gyroc improve over good old fashioned lead in any particular way? The answer is that it depends. A gyroc accelerates up to the max speed it can manage and stays there until it runs out of fuel, meaning it stays at full power for a good long while. Larger caliber gyrocs would essentially be improved slugs, and comparable to the use of shotguns in tactical situations: Very capable but limited by sheer size and weight of ammunition. Much like a shotgun for military use, they'd be likely used as what they are: a tiny rocket launcher (the slug or 12ga grenade filled shotgun being a tiny smoothbore cannon), delivering minigrenades with accuracy and range, possibly mounted to a carrier firearm. Depending on the effectiveness of the gyroc, the function of the gun to fire it, and comparisons to standard ammunition (price, weight, size) small caliber gyrocs could well take the place of the traditional round as the standard. That said, traditional rounds will never go away (I'd hardly be surprised if 9mm lasts as long as the species and possibly beyond)