From the back of the book:
Guns!, guns!, guns! (3G³ for short) lets you design custom weapons for virtually any roleplaying game. Design realistic weapons in a universal format, and convert the stats to the system you like best, or even between different systems. In addition to the overall conversion guidelines, there are now detailed conversion notes for Corps, TimeLords, MegaTraveller, Cyberpunk, TORG, GURPS, Twilight: 2000, and the HERO system.
Pick your weapon! You can choose from conventional weapons, rockets, lasers, particle beams, railguns and melee weapons. Detailed directions and step-by-step design sheets help guide you through each aspect of custom weapon design.
Design weapons for any Tech Level where they are possible, not just where they historically appeared. Want to give the Romans machine pistols? Or design lasers for WWII? If it could have been done, or if it might be done, you can do it, in 3G³.