So I did a little search at Tutorials-section, and didn't find any tutorial for San Andreas for file weapon.dat.
So let's begin, shall we?
A: Identifier
Used to easily identify weapon, can be present multiple times for some weapons as they require multiple entries because of weapon skill levels.
B: Fire Type
Defines if the weapon will have regular/instant hit bullets or sniper bullets, or if they are projectiles or area-effect weapons, for example grenade is projectile and flame