Zozer's Modern War Weapon Conversion is prolly the best for existing weapons.
Now if you want to create new realistic "real world physics" weapons...
Fire, Fusion, and Steel and
Guns! Guns! Guns! v1.1
Fire, Fusion and Steel was the Traveller: The New Era equipment design book. Well mostly Vehicles and Starships. but there were design sequences for personal unpowered (archaic) armor and personal weapons. The numbers click for its system, but you can use it as a guide for Cepheus, or it in conjunction with the Zozer book
BTRC's Guns! Guns! Guns! is my favorite. I use it as my Rosetta Stone for weapon conversions. It allows you to design all characteristics of weapons starting with the bullet/warhead/beam/etc. Then you design the weapon itself, range finders/stocks/magazines. Tech Level differences/efficiencies are factored in. Once you are done you have a set of stats which allow you plug in directly to TimeLords and CORPS (BTRC games), GURPS 3rd Ed, Hero Systems v1-4, Cyberpunk 2020, Heavy Gear, Battlelords of the 23rd Century, Nexus, Feng Shui, Traveller: New Era, Traveller 4th edition, FUZION and Masterbook (yea a whole bunch of '90s games, still...) and a practical section on creating conversion to other systems (FGMP/PGMP in Storyteller System or GUMSHOE anyone

LOL!). Bows, slings, and melee weapons too
Fire, Fusion, and Steel at DriveThru
Guns! Guns! Guns! v1.1 at DriveThru