In GTA San Andreas, you can upgrade your skill and proficiency with each weapon. Weapons can be found and picked up from various locations around the San Andreas map, and several of them can be purchased via Ammu-Nation (prices are 20% higher in Las Venturas). This complete GTA San Andreas Weapons Guide includes all Guns, Rifles, Explosives, Melee, and every weapon featured in the original game, as well as in GTA San Andreas: The Definitive Edition.
On this page you find the full list of weapons in Grand Theft Auto: San Andreas.