In addition to fully upgrading these weapons, you must also purchase the unlockable weapons from the Item Management Menu.

Unlockable	How to unlock
Gatling Gun	Fully Upgrade VZ61 Machine Gun.
Hydra Shotgun	Fully Upgrade M93 Shotgun.
Longbow	Fully Upgrade S75 Rifle.
M93R Handgun	Fully Upgrade M92F Handgun.
S&W M500 Magnum	Fully Upgrade S&W M29 Magnum.