I build 2 or 3 rifles a year for myself.
I started 25 years ago hitting the pawn shops at lunch and buying every broken rifle I could find. I would buy parts and Numrich and sell the fixed rifles on consignment at other pawn shops.
What really gave me a boost was when I loaned someone the money to build a machine shop. The documentation showed all the stuff I had never heard of; surface plate, height gauge, di-chem, parallel bars, a big lathe, a knee mill, welder, pin gauges, gauge blocks, depth gauges, dial indicators, magnetic bases, test indicators, fly cutters, boring heads, optivisor, angle plates, rotary table, collets, 115 piece premium drill sets, drill and tap chart on the wall, copy of Machinery's handbook, DRO, etc.
I started reading the machinist forum, rec.crafts.metalworking and practical machinist, and home shop machinist.
I have dozens of books on gunsmithing. I buy a lot of parts from brownells. I talk to the tech advisers at Brownells. They know a lot about reamers and paint.
I went to a hole in the wall gun store run by a gunsmith, bought a pound of powder and asked a stupid question or two about truing rifle actions.
There were some great posts on The Firing Line 15 years ago by Gale McMillan [a rem700 and benchrest guy].
There were some great posts on rec.guns 20 years ago by Bart Bobbitt [A Win M70 and high power competitor at Camp Perry].
When ever I see John Benjamin [barrel maker and high power competitor] at machine auctions, I ask him stupid questions.
I started writing up range reports like lab reports to figure out what was working and what was not working for accuracy.
I started handloading in my vehicle for long range target practice.