Howie,,,,,,
I'm a retired law enforcement officer and during my career I worked often with probation and parole officers and on occasion assisted them in their duties which included confiscating weapons that the person on probation was prohibited from possessing. That being said I can tell you that a probation/parole officer has the legal authority to confiscate items from a parolee or someone on probation if the item violates the terms of the probation or parole,, that certainly includes air rifles.
That being said I can tell you that any items confiscated by a parole/probation officer is fully documented in his/her incident report along with all the pertinent information concerning the incident. Part of that official documentation would also include invoicing the item (airgun) for safe keeping as evidence of the probation violation so that it would be available should the probation violation matter wind up going to court or a judicial hearing.
My suggestion is this......... if you have the residential address of the person you shipped the rifle to then it should be a simple matter of finding out the probation department involved in the case. Give them a call. Provide the name of the person you shipped the rifle to and tell the probation dept. that you need to speak to the probation officer who is charged with overseeing the probation for that individual. They will be more than happy to connect you to the proper probation officer. Discuss the matter with the probation officer. He should be able to tell you what procedure you have to follow to have your air rifle returned to your custody,,, however keep in mind that the air rifle may not be able to be returned to you until the probation violation matter has run it's course. You will of course need to be able to prove ownership with some sort of sales receipt, warranty registration, etc......
As a side note..... I have no doubt that the probation officer will be VERY interested in the fact that this person has purchased other air rifles in the past, these prior purchases also being a violation of the conditions of his probation and a matter that the probation officer will no doubt be very happy to investigate.....
Hope this helps you out some.....
Jeff