Ooo, I was playing Battlfield 2142 today and they're right, they are believable weapons. I have recently been making some sci-fi weapons for FPSC and this is my advice.
Think about what material a gun in the future will be made from. Wood is a no no so stick to metals and plastics. Most sci-fi weapons look shiny but to make believable guns, don't make them look naff.
Think about electronics and lights. Ammo counters and other numbers on little LCD screens give weapons that futuristic look.
Think where the magazine would be situated on conventional firearms then move it around to a new location like behind the trigger, in the hand grip, to the side of the gun or on top.
Put some new grips (vertical or horizontal) around the stock.
Add some complicated looking optics or scopes.
Make the gun look heavy. Small flimsy looking weapons don't feel as futuristic.
Design a new charging handle or system, like a pneumatic hatch that opens so you can slide a magazine in.
Use different sounds like pneumatics etc and gun sounds with and added sci-fi kick.
Have the gun use different ammunition like plasma or something.
Give the muzzle flash a different colour like red, green or blue.
Hope this helps.
