this is a serious offense and you could be facing jail time, even if it is your first offense. The best way for you to defend yourself and attempt to clear yourself is to hire yourself an attorney and do so as quickly as possible
In order for you to be found guilty of being in possession of stolen property the State has to prove that you knew, or had reason to know, that the property (in your case a firearm) was indeed stolen. The facts of your question indicate no such knowledge on your part. Inasmuch as your story (if such is what actually transpired) demonstrates no culpable knowledge, such veresion of the facts, coupled with your lack of any criminal record, should make valuable points for a presentation by your attorney to the prosecutor to dismiss the charge. I strongly urge you to undergo a non-stipulated polygraph examination by a reputable and qualified polygraphist, which examination should be set up by your attorney. Should the examination support your story, your attorney will then use the test result to advocate on your behalf with the prosecutor. If you have not retained an attorney you should do so now and have him fight for you. If my office can be of assistance to you, do not hesitate to contact us. Good luck to you.