If your friend who has been convicted three times for crimes, wants you to sign an affidavit stating that you own the gun that he is charged with possessing, I would not sign the affidavit it the gun is actually not yours.The issue I see is not whether you own the gun at issue, but the fact that your friend was not supposed to possess the gun he is charged with possessing. If the gun he is charged with is a hand gun, most likely it is registered to some one, and if not, it is required to be registered. If the hand gun is not registered and you claim it as your own, then there could be fall out for you concerning it.My suggestion is that if the gun you are writing about is not yours, I would not sign any affidavit about it. If you do, you very well will be exposing yourself to possible criminal charges.