Hi Charley,
Probably 99.99% of maple stocked long rifles are stained and then finished with some sort of oil-based varnish. Unless the stain the guy used contains finish, you should have no problem whatsoever putting your favorite oil based finish on top of the stained wood. I lived in SE Alaska for over 20 years, a place that makes coastal Washington and Oregon look like dry and sunny Montana. I urge you to rethink your favorite linseed oil finish. Let me suggest an alternative. If you insist on linseed oil then mix 25% linseed oil (hardware store boiled linseed or raw oil is fine), 25% polyurethane varnish, and then 50% mineral spirits or turpentine. I would substitute tung oil for the linseed but linseed works OK. You can use that mix to simulate the appearance of any original long rifle low to high gloss and it is far superior to boiled linseed oil with respect to weather resistance. I would still be using that mix with tung oil if it was not for the fact that I can now buy Sutherland-Welles polymerized tung oil, which I prefer.
dave