You don't find the weapon address but the address to slot, then you just write the weapon ID to this address and also the ammo value otherwise you won't see it ( ammo addy is slot addy + 12 and its not a pointer, you just add 12 to that address.
There's a website with gta sa addresses, I'm on my phone so I can't give you now. There's a list of weapon slots & IDs that go to them ( you can't spawn minigun in other slot than heavy weapons )