i have iphone 3g 8gb ... 3.1.2 with firmware 5.12 bb 6 ... i try to reset all setting option from that time my iphone is struck on start-up apple logo ... any tip will be highly appreciated ... it was jail-broken with red-snow ....

Restore it with itunes using the correct firmware
and for your clearance, 3.1.2 is the firmware and 5.12 is the Baseband

and if u are sure u have 5.12 baseband then u won't be able to unlock your device.