Subscribe to download
Borderlands1 Gun Sounds mod

Changes most of the gunshot sounds in the game to those from the first Borderlands. Includes incindiary variants.
Changes:
Pistol + DualPistol
Magnum
SMG
Silenced SMG
Shotgun
Chrome Shotgun
Auto-Shotgun
SPAS
Assault Rifle
AK-47
SCAR
Hunting Rifle
Military Rifle

Does not change:
Melee weapons
Turret weapons
Counter-Strike weapons
Throwable items
Grenade Launcher
50 CAL

I whipped this up when the first Borderlands came out, and I've been using it ever since, so there shouldn't be any issues.
Sounds for Dual Pistols sound the same because of the way L4D2 plays the files - your starting singular pistol would change its fire sound when picking up a second one of a different model, and the new one would play the old sound. Thus, problem solved.