Gents,
As with fighter jets, human physiology is the limit here and I'd say we'd pretty much reached it already with handguns and small arms. Newton's Second Law can't be cheated. There is only so much recoil the human body can take while still using a firearm effectively.
There are various bells and whistles still to be developed, aiming geegaws, breach improvements, ammo tweaks, stability aids, weight reduction/distribution, and so forth. However, the limits due to recoil - a product of projectile mass and muzzle velocity - have been reached. There are already handguns on the market that no sane person would want to use regularly and no one but experts can even fire safely. (Desert Eagle I'm looking at you!)
Even Traveller's vaunted (and fictional) gauss weapons face the recoil hurdle. It doesn't matter if the projectile is a tiny sliver, at those muzzle velocities there is going to be major recoil.
Self-propelled projectiles can break this 'recoil barrier' but a lot of work needs to be done with them. Thanks to a link on a weapons discussion board earlier this year, I read a report of a fairly recent Gyrojet test firing in a small arms magazine. Although originally printed, the report was on-line.
The reviewer had much to say about the poor mechanical 'fit & finish' of the pistol itself. He had even fewer goods things to say about the quality of the Gyrojet's ammunition. Burn time, thrust, and other factors varied greatly from round to round. This, plus the poor manufacturing of the pistol itself, understandably effected accuracy a great deal. As far as accuracy was concerned, the reviewer rated the Gyrojet well below even bargain basement pistols in the same calibre.
Naturally, all the faults he found could be remedied by an improved manufacturing process but, at it sits now, the relatively few Gyrojets made in the 60s are worth more as collectible oddities than as working firearms.
Of course, in the 57th Century self-propelled and even self-aiming(!) small arms projectiles may be the norm.
Have fun,
Bill
