It depends on how the coats are applied and how thin they are. That also affects how tough they are.
For barrel channels and such your three coats are probably ample plenty if they were brushed on. And in fact brushed on coats are fine for such inside places, but lousy for the outside. They just never get very hard and wear off easier on outside surfaces.
For the exterior, it's best to put on the first coat heavy, then give it 24 hours or so to dry in a warm place. Steel wool it back down almost to wood, or actually with a little wood showing, then do it all at least one more time again. That's the filler.
Once the filling is done, I give a stock a full 48 hours to dry and harden in a good warm spot before actually starting the "finish" coats.
These I put on one drop at a time. Basically dip the very tip of a finger in the bottle, then spread it out on the stock till it won't spread any more and gets tacky. At that point, it pays to go ahead and rub it some more so it starts feeling hot under your hand to "stretch it." Keep doing that over the whole surface of the gun and you have one coat. Yeah, you can work up blisters when you do it right.
Let that coat dry 24 hours in your warm spot, steel wool it very lightly with 4/0 and then stretch on another coat of finish.
Depending on how much you filled the grain on your first fill coats and how much grain you want left for texture, I usually figure on a minimum of six of the thin "stretch" coats and 8 or 10 are a better minimum most of the time.
Even doing the stretch coats, the finish is going to be a little tender and subject to wear for a while. Given a week or two in your warm spot to cure it is going to be reasonably tough, but I don't figure it's tough enough for hard treatment until the gun has been in your rack a month or two.
I've got quite a few guns finished this way over the years that never seem to wear. But I've seen thick brushed-on TruOil finishes that are still soft years later once you break through the hard surface layer.
BTW- I just did my first stock using tung oil finish a week ago. You had to watch the drying between coats a little closer, but I got the finish I wanted a lot quicker than with TruOil, and it's already a whole bunch harder and tougher. After closely comparing it with my pride and joy TruOil stocks, I went in and threw away all my bottles of TruOil. Been there. Done that. Got the T-shirt. And won't ever do a TruOil finish again. They say wisdom comes with age, but some of us just have to be a lot older to find it.
