i rub the oil in like shoe polish for the first 5 or 6 coats, then, depending on the grain of the wood, i will use 1500 grit paper and oil to work more oil into the grain. i have found this to be the easiest way to fill in the grain. once the grain is filled, i sand flat the rifle with 1500 grit paper, using water as a lube with the paper. once sanded flat, oil is wiped on with a piece of cotton t-shirt
then let dry. add one coat at a time untill you are satisfied with the depth of the finish. allow a couple of weeks for the oil to cure fully, then polish. as to the durability of tung oil, i have in my collection a 7.7mm arisaka that was sportered in the 60's by my father, that rifle sees 4 to 6 weeks in the woods and 10 or 15 trips to the range a year, both in his posession, and mine, for more than 40 years, and the finish is holding up very well.
Hundreds of years from now, it will not matter what my bank account was, the sort of house I lived in, or the kind of car I drove... But the world may be different because I did something so bafflingly crazy that my ruins become a tourist attraction.