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If you're sure you have a legitimate PSN copy you own and need a .rap to activate it, the only legal way is to extract it from your own PS3 system using RAP Extractor or PS3ExportRAP .
It sounds like you're asking about a for Metal Gear Solid 4: Guns of the Patriots (typically for PS3 emulation, such as with RPCS3 ). mgs4 rap file