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It depends much on the State where you live.
If you want the maximum documentation, have the transfer done by a firearms dealer (though he will certainly charge you between $10 and $50 for the service).
Going to web sites like "Legalzoom" or do a Google search for "Business forms-Bill of Sale" will get you some text to put in a document. Also office supply houses sell blank bills of sale.
The particulars that I (off the top of my non-lawyer head) are that the price (or trade items), date, serial number, description of the gun and all accessories coming with it, the buyer's and seller's names and perhaps identifying features like Drivers License numbers. Also any other terms, like warranty (if the seller guarantees functionality) and sometimes a clause certifying the owner's right to sell the item. This last item always seems to me to be overkill, though.
I just use date, item description (complete), price, seller's name, buyer's name.
Copies for both the seller and the buyer.
Lost Sheep