[Martinjmpr]

KCPhoenix: The legalities of him owning firearms are between him and whatever entity tries to prosecute him (if any.)

Your issue is simply reclaiming your property, which he has improperly converted to his own use ("Conversion" being the civil equivalent of theft.)

Therefore: Sue him in civil court in the district in which he resides. You will probably have to hire a process server. Given the amount of the property involved, you probably won't be able to do it in small claims court although you should check (here in CO I beleive the maximum amount in question has to be under $7500.)

Given the facts I would say consult a reputable lawyer in your area, the lawyer will be able to tell you how much it's likely to cost you and how likely you are to prevail. Expect to have to show proof that you purchased or owned the firearms in question. Remember that the burden of proof is on you as the plaintiff.