I haven't worked on Nevercast in weeks. I have to do a whole bunch of boring research on the properties of firearms, and I'm also concerned that armors of the future (such as carbon nanotube designs) will surpass infantry weaponry. So, I'm having a hard time concieving of what skirmish combat will be like in the future. I don't want it to be like Dune where the arms race collapses upon itself and everyone is fighting each other with swords on spaceships.
I've hit a plateau here...
Rail Guns are the ish
But they won't be prolific in the Nevercast setting, and I suppose neither will carbon nanotubes. Actually, I thought it would be fun to use a giant rail gun as a futuristic train system and it would be powered by Tesla towers.
To clarify further, Nevercast is neither a space opera nor cyberpunk. There are no mega-corporations, and there's really no space-travel (maybe some abandoned space facilities to raid). This is because the Nevercast caused a great deal of social decay, so many technologies are dead or progress very slowly, including consumer technology, mainstream media, space travel, and materials technology. However, I believe medical technology will still be strong, and "green" technology because of its efficiency. Quantum computing in this age is new and wickedly powerful, and the first A.I. named "Algorithm" was constructed using it (inspired by Project Blue Brain). Robotics is catching up.
On a side note, I would like to point out that the task resolution system in Nevercast is not binary. If something is beyond your attribute, it does not mean it is beyond your skill, it means you have to put effort into the task at hand, or concentrate upon it. For example, if you instantly raise your rifle to shoot at something 100 yards away, you will most likely miss (the task surpasses your focus attribute), but if you spend a few moments to aim (i.e. combat actions), you can hit it. In game terms, this is represented by a dice roll (d4, +1 to +3 on your attribute), in which your dice can "explode" (on a 4) enough times for you to make a low-probability success. These mechanics, I believe, make for a more realistic exchange in which combat time is extremely delicate. To illustrate, when I'm boxing a fast opponent, he is almost
impossible to hit with any combination, but that does not mean he his unhittable. When I spend a few moments to match his rhythm, I can realize a pattern in his movements and catch him repeatedly.
Now on to the point. I was hoping you would be the first to respond because your insights are extremely helpful. I had forgotten about the non-lethal electric weapons, which I believe will be prominent tools amongst police forces in the future.
Also, conventional war is not common in the Nevercast universe, but skirmishes are as well as subterfuge. As the populace is more heavily centralized in this future, this combined with widespread social decay will render lawless a great deal of the landscape.
If I could describe the genre or feel of the setting...
Then I would say Nevercast is "Romance of the Three Kingdoms 2065". Now, I've never read "Romance of the Three Kingdoms", but I like the idea of masterful scholar-warriors intertwined with complex political intrigues. However, I didn't want the advanced martial arts / meditational practices presented to give the impression of magical ability, or seem hokey like in the Matrix. I want it to seem mundane - like these feats can actually be attained by humans in the future, who have greater mental capacity, augmentive biological technology, exponentially better training methodologies, and (in this alternate reality) have institutionalized these arts for millenia. It's sort of a hypothetical question like, "What if Shaolin Temple never burned?"

Don't know the way from here.
I'm assuming that the firearm properties that you care about are basically length, weight, kick, and range. How bulky is the weapon, how quickly one can aim it, how much it tends to go off aim when fired, and how much it tends to go off aim at far away targets, and then you can use the basic Nevercast compare attribute versus how difficult a shot should be based on speed, range, cover, visibility, etc. So most shots could then just be "you can make this shot", or "you can't make this shot".
Given that, I have almost no clue where to find these properties. I think I might look in Palladium's book of Contemporary Weapons for weight, length, and range.
For future combat. The reason Dune was that way was because it was to an extent a fairy tale, and all space opera is.
Think on this. In one morning in the fall of 2001, a handful of men killed more than 3000 Americans using normal construction tools and transportation technology. As our technology advances, this will only get worse. When we have interplanetary technology, our normal construction technology will involve moving asteroids around to harvest resources from them. Moving asteroids have periodically caused the extinction of the majority of species on planet Earth.
It is extremely unlikely that human beings will be able to continue to have wars in "the future".
But if we want to think about possible space opera war or space opera cop stories (technological fairy tales) I think that the weapons of war will become so deadly that no armor will be useful. Vehicles will have to rely on speed and maneuverability, and people will have to rely on stealth. I don't care if your armor is impenetrable. If I can get an iron slug going fast enough out of my magnetic accelerator rifle, I can splatter that iron slug across your chest so hard it will liquefy your organs.
For space opera cop stories, judging based on how much cops absolutely love tasers and will taser anyone for almost no reason, I'm going to have to have to guess that more non-lethal weapons are on the way. Cops have horrible jobs. It is boring with short bursts of terrifying danger. Things like traffic stops or domestic calls can become deadly in less time than it takes to sneeze. If a cop can shoot you with something that will probably not cause lasting harm to you, he will. They love love love that they can pull a weapon without thinking about people's lives. More of those are on the way.