My first was built from plans I saw in (I believe) Mechanix Illustrated magazine back in 1963 or 1964. Dad would never have permitted me, at 14, to buy a rifle and I had little money anyway. When I read the accompanying article it included sources for parts, specifically a 2-groove 1903-A3 Springfield barrel that could be had for the princely sum of $3.95! My paper-route money could stand that cost.
A lock was available from Dixie Gun Works for $8.95, and the trigger and trigger-guard probably $2.95. The breechplug was a grade 5 machine bolt; the drum and nipple were fabricated out of stuff I scrounged from Dad's little shop. A neighbor gave me a chunk of Philippine mahogany which I butchered for the stock, and the buttplate came from a junk WWII Japanese Arisaka training rifle.
With absolutely NO experience (or skill) in wood or metal work, I glued, screwed, and nailed all those parts together into something that resembled a rifle - if viewed from 25 feet away. It was so rough that to call it crude would have been an upgrade. I got it to shoot so well I could guarantee that it would hit the ground with every shot.
That rifle is long gone now, replaced by some genuine for-real muzzle-loaders, after having given me an interest in black powder shooting that has lasted over 50 years thus far.