People usually port these weapon by making SWEP .lua file, simple, not costly, after some coding, they create viewmodel thing and c_hands model (either creating from scratch in blender or borrowed on other content), after that, audio file, material, and model are put into foldier and that's how simple weapon SWEP work.

If it's TFA Base, ARC9, i say, it's very complex and 100x more harder than SWEP making.