First, do you want to keep the guns, or sell them? If you want to keep them and use them, then the rust is not too important as only you care about it. Some cold blue might work OK to just cover up the worst spots. It won't last, but it is cheap.
If you plan to sell the guns, then I think that spray on coating, like paint but better, would be the way to go for the shotgun.
If the only rust on the rifle is on the bolt handle, I would just take the rust and the rest of the bluing off with a rust and blue remover or emery cloth, then polish with a fine (600 grit or finer) oil it and leave it. Many rifles have bright bolt handles, so it won't look odd.
Jim