Credit to erod550 for his guide at
http://www.xbox360achievements.org/game/g-force/guide/Purchase all new weapons and weapon upgrades
As long as you have gotten all the golden disks up until then, you will get this after you get the Nano Hack v2. You do NOT need to have all of the ammo upgrades for this, just the guns themselves and their upgrades. The golden disks always have a cut scene when they appear so you should not miss any of them.
List of Weapons in the game:
Electro-Whip
Electro-Whip v2
Saberlizer
Plasma Gun
Cluster Rifle
Cluster Rifle v2
Shot Bolter
NanoHacker
NanoHacker v2
Magnetic Grapple
Freeze Gun
Flame Thrower