Hi Frank,
Shortening the barrel will almost certainly reduce the gun's value as an antique. A knowledgeable buyer or collector will use that fact to drop the price he or she is willing to pay. Certainly, the owner can do whatever he wants and shortening the barrel as much as the OP suggests will lighten it a lot and it will be part of the gun's history. However, that does not change the fact that as a historical object, the value will be compromised. It would be nice to see the rifle and identify it if possible before it is altered in any way.
dave