Ooooh! You screwed up by not contacting the builder right away. Bad mistake! But, the problem may not be as bad as you think. If the rust is confined to the exterior of the barrel, the gun is quite salvagable and is likely going to be a good shooter. Take the gun apart and carefully examine the lock, the triggers, the stock as a whole and carefully examine the barrel channel of the stock for any signs of cracking that may have been caused by the expansion of the rust on the barrel. If it is okay, the rust on the barrel did not do any damage to the stock. Now, examine the bore. If it is not badly rusted, your only problem is the rust on the outside of the barrel. That may not be so much of a problem as it appears. Having set this long, the rust has most likely penetrated deeper than one would want and the surface of the barrel could be pitted to one degree or another. Even this is not a complete tragedy. Just grab some 0000 steel wool and some motor oil and start scrubbing. The 0000 steel wool, when used with a good oil, will not cut into the hard browning that is under the rust. Use plenty of oil and scrub the manure out of it until you have removed all of the loose rust. It's just possible, but don't hang your hat on it, that under that loose rust may lie a nice looking browning job.
If the rust is confined to the exterior of the barrel and scrubbing with steel wool and oil doesn't solve the problem, you can always use some emery cloth to polish the barrel back to clean metal and then use some Laurel Mountain Forge browning solution to re-do the browning on the barrel. It isn't all that hard to do, it is just applying the browning solution to the barrel and letting it rust. Then you scrub off the rust with water and some kind of rough cloth such as burlap or terry cloth and apply another coat of the browning solution. Keep doing this until you have the depth of brown that you want. At that point, you scrub it again with water and rough cloth, neutralize the browning solution with baking soda, dry it off and oil it with a good grade of oil. Then you must let it sit for a few days to let the browning (rust coating) harden. Finally, rub it with some rough cloth and more oil to remove any remaining soft rust and it is ready to go back into your stock.
All may not be lost. With a bit of care and a bit of work, you could still end up with a fine rifle. :thumbsup: