There is a research paper from the US Forest service in which they detail the results of durability test of various wood finishes. The results show that the most protective finish is epoxy and the least protective linseed oil.

But protection is not the primary goal when finishing a gunstock. Which is why we turn to linseed oil, over wood sealed with varnish or sealer etc, to get that unique feel. Guns are handled a lot, and the tactile characteristics matter.