I know for a fact that the PS2 version, the weapons you can obtain with unlimited ammo are:
1. Infinite Launcher (Obtainable after beating the game once. 1,000,000 Pesetas)
2. Chicago Thompson Typewriter (Obtainable after beating Separate Ways. 1,000,000 Pesetas)
3. P.R.L. 412 (Obtainable after beating the game on Professional Mode. 0 Pesetas)
4. Handcannon (Obtainable after getting a 5 star with every character on every map in the Mercenaries Mode. 0 Pesetas on initial purchase, 790,000 Pesetas to fully upgrade to infinite ammo.)