Rust on the outside does not always mean rust on the inside. 0000 steel wool and some gun oil or CLP will remove rust colored stains. If it has pitted the steel nothing will fix the pits.
Get a cleaning kit and scrub the inside real good with solvent ( I prefer Hoppes ) any gun cleaning solvent should work. Then run an oiled patch down the barrel. Run a dry patch down the barrel before shooting it.
And yes it should be safe to shoot. If there is pitting rust or if you have any other reason to doubt the integrity of the gun, take it to a gunsmith to check it first.
You should be able to find an owners manual for your model on-line that you can download for free. It has the direction for cleaning and lubricating vital parts. If a search engine doesn't turn one up contact Mossberg directly. They will help you get one.
ETA New Haven is used on several models there should also be a number associated with the model. That will help your search for a manual. "Mossberg 250" will get the best results.
You can't fix stupid....however ignorance can be cured through education!
Last edited by big al hunter; June 23, 2013 at 10:00 AM.