Working on Singleplayer.
Extract C:\Program Files (x86)\Steam\SteamApps\common\Call of Juarez - The Cartel\MD\Data0.pak with 7z or WinRAR.
Find file WeaponsUnlock.scr, change extension from .scr to .txt.
Open it. Find "Firearm_PpcScope", "Firearm_P99" and "Firearm_Tmp", set last position (extra unlock) -1. Also set value high than 0 for map or level position. Like that:
Weapon( "Firearm_Tmp" , 0 , -1 , 2 , -1 );
Save it. Return extension .scr. Put weaponsunlock.scr in your user directory Documents\Call of Juarez - The Cartel\out\data.
Done.

You may also add grenade launcher in "load-out" arsenal. just paste this line:
Weapon( "Firearm_M72Law" , 0 , -1 , 1 , -1 );