Without more information no one can tell you why the feds put a detainer on him. What it means is that when is about to be released from local custody federal marshals will pick him up at the jail. He needs an attorney.
The Feds are not precluded for charging him for the same crime he was arrested and charged for in state court. You should speak with an attorney about whether your husband may have been charged federally for the same firearm.
Please don't forget to choose the "best" or most "helpful" answer. This response does not create an attorney client relationship. The information provided here should not be relied upon and is not a substitute for legal advice received by an attorney that you have retained for your specific case. If you are actually involved in a case pending in court and you have an attorney, you should speak to your attorney and follow his/her advice. The information provided here is for general purposes only.
Many times the State will drop the charge and inform the Feds if they have jurisdiction of the crime because their sentencing may have a harsher penalty. But, you need to tell us why he is being detained. Is it the same charge or another one? You should have a private lawyer or public defender on the State's charge, so they should be able to tell why there is a hold oh him. You should contact them immediately.