can someone please tell me the procedure for importing and registering a small weapon in Nigeria,for private use.

You would need to be a serving or retired Nigerian military serviceman with mandatory individual presidential permission in order to qualify for a licence to carry or keep a pistol. 

You can, however, get a licence for hunting guns from the police, if you know someone to do the usual legwork and palm-greasing.  Good hunting, and always carry cash to submit with your weapon permit, should you travel "heavy" and run into the man.