Wolfenstein

To upgrade your weapons just visit a black market dealer when you are not on a mission. There is also one at the end of the game in the Zeppelin level.

The easiest gun to upgrade is your MP40. It is available from the start of the game & I think it is one of the cheapest to upgrade overall. When the last upgrade has been bought the trophy will lock.

See below for a full list of the guns, the upgrades available for each gun & how much they will cost you:

MP40:
Recoil Compensator $1000
Silencer $500
Drum Magazine $500
Big Bore $2000
Improved Rifling $1000

Kar98:
Improved Rifling $500
Silencer $500
Stripper Clip $1500
Big Bore $1000
Sniper Scope $2500
Bayonet $500

MP43:
Improved Rifling $500
Drum Magazine $500
Ammo Pouch $750
Big Bore $1500
Technical Scope $3000
Recoil Compensator $1000

Panzerschrek:
Light Weight Build $1000
High Explosives $500
Ammo Pouch $500
Stabilized Rockets $500
Magazine $2000
Seeker Crystal $2500

Flammenwerfer:
Improved Fuel Pump $1000
Expanded Tanks $1000
Carburetion $2000
Pump Overdrive $500

Particle Cannon:
Particle Lens $1500
Veil Catalysts $500
Improved Veil Capacitor $1000
Beam Intensifier $1000
Flux Arc $2000

Tesla Gun:
Upgraded Battery $1000
Step Transformer $1000
MKZ Generator $500
Arc Nodes $500
Coil Upgrade $2000
Current Induction $2000

Leichenfaust 44:
Gallium Transformer $1000
Nacrite Resonator $1000
Planck Refractor $2000
Iridium Canister $1500

Mdl. 24 Grenade:

High Explosives $3000
Ammo Satchel $2000
Fragmentation $1000

You DON’T need to buy the Ammo Refills for your weapons. All they do (as the name suggests) is refill your ammo to the maximum., they aren’t an upgrade.

You can keep an eye on your upgrades by pressing and going to the ‘Upgrades’ tab in the menu.