I am not new to guns, but I am new to buying gun parts kits. I own an AK-47 and was wanting to buy some extra parts for it. As I searched around I have seen some parts kits that include most of the parts of the gun except a few, like the receiver. The kits look like guns that had been broken down into multiple parts for sale, like the gun had been decommissioned. What is a gun parts kit exactly? Thanks.