No, it is not part of the unmodded game.
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All you have to do is delete your .minecraft folder and run Minecraft agian. It will make a new one, then try installing it agian.
If it is the classic version that is free anyway on the minecraft.net site it is not illegal. if it is the proper minecraft that has survival and you download it free it is not illegal BUT it is illegal if you play it without a payed account. just because other people have it it does not make it illegal.
There is no brass in MineCraft without mods.
Not in the current Vanilla (Minecraft without Mods) Minecraft, no. But their are a few Mods out there that will make that possible.
Guns can't be made in Minecraft without modding.