I think plausible SF guns should be either current gun designs with new functional improvements, or new designs that look more efficient or deadly and somehow incorporate "new" technology. Of course, some of the design that goes into firearms is aesthetically motivated...tastes would change, too.
If there was a new chemical that replaced our current smokeless powder technology, it would affect the overall size of the cartridges a lot. Likewise, a better expansion system would make the projectile look different.
New materials, alloys that would be lighter and stronger, could allow for more minimalistic designs.
New sighting systems could allow weapons to "lock on" and stabilize the arm/hand despite body movement, before firing.
Based on movement patterns of the target, material being fired at, and destructive needs, a weapon could assess the situation and "cycle" to the appropriate round. Keep in mind that with microchip technology, smaller projectiles, and improved mechanical design, a weapon that's the size of current weapons could do all this.
There could be weapons that explode when captured, or detonate a themonuke when the owner dies.
So far the discussion here has been mostly about aesthetics, but I don't think tacking accessories onto current firearms is the way to go for "hard" SF, especially when it takes place many years from now. More fantasy oriented SF, and especially SF that just needs a cool looking weapon to please an audience, can resort to the "current weapons"...but I think cool, plausible weapons concepts that fit with a changed world would add a lot to an already good hard sci fi novel or movie.
