You might want to check with someone familiar with NY law or contact the NY State Police. It may very well be illegal for you to send it to NY for transfer or sale. NY has banned the sale and I believe transfer of "assault weapons" as defined by the NY SAFE Act. In order to keep these rifles, they had to have been registered. They either had to be registered or sold/transferred to an out of state person or NY FFL holder. I'm pretty sure it would illegal in NY for you to send it to him. That plus it being declared abandoned property, I'm pretty sure that it is yours. If he plays hardball with you, ask him to send you a letter from the NY State Police stating that it is legal to send this rifle into NY for transfer. That'll never happen and will shut him up.
The NY SAFE ACT:
"Broadens definition of "assault weapon" from two identified features to one. The sale and/or transfer of newly defined assault weapons is banned within the state, although sales out of state are permitted. Possession of the newly defined assault weapons is allowed only if they were possessed at the time that the law was passed, and must be registered with the state within one year."
This would seem to prohibit you from transferring the rifle back to him.
