You have to upgrade all the weapons so you can get the achievement
you don't have to beat the game, you just upgrade a weapon completely, go to bonus features, and look for that type of weapon you upgraded and you buy it. It will say for example: M29F INFINITE AMMO. All infinite weapons you upgraded will be at the bottom 4 you to buy.
His weapon is the Samurai Edge 2
The wunderwaffe DG2 especially if it is upgraded, it can kill at least 15 zombies with one shot when upgraded.
If you are referring to upgraded ammo, as in when you see upgraded Ammo on the walls, then. Upgraged Ammo is simply ammo for a Pack-A-Puched Weapon. For example, the MP40 has ammo that cost 500 points, when Pack-A-Punched, it will cost 4500 points to replenish the ammo.
You get the most awesome weapon in Re4. The Handcannon. Once upgraded fully it has unlimited ammo and will be as powerful as the Chicago Typewriter. One hit kill for all zombies!
The Infinate Magnum
The weapon with the most raw power is the Seismic Gigas Hammer. It is upgraded from a Bone Hammer.
Crusher Hand, especially when upgraded to the max
Complete the indicated task to unlock the corresponding weapon or skin. Note: Achievement-based bonuses will not activate until the day after earning the achievement.
Read the game manual.
To get the 'Maxed Out' trophy/achievement. You need to fully upgraded ALL weapons and equipment. To upgrade a weapons or Isaac's RIG, you will need power nodes which are scattered around the Ishimura on the walls or dropped by enemies. You can also buy them at the Store for 10,000 credits. Then you will need to go to an upgrade bench. When you have upgraded EVERY weapons and ALL of Isaac's equipment you will get the trophy/achievement. You shouldn't fully upgrade a weapon and then sell it and upgrade another. Note: it is impossible to do this on a single play through. You will need to start a 'New Game +' which will carry over your equipment and weapons from your previous play through.
Depends of what weapon you want. If you want a Member weapon, then you have to get your account upgraded. If you just want a another weapon, fight mosters. Monsters some times drop weapons.