Airplanes. Really.
I don't fly anymore but in an earlier life aviation consumed me. Had a company plane and during time off I stole seats in all kinds of homebuilts, military stuff and ragwings.
I had shot occasionally with friends and knew the bare basics but bullet type, velocities, expansion, etc was all greek. Then one day my gun enthusiast friend, who also flew, switched my light bulb... bullets (especially expanding ones) were like airplanes. They had a minimum and maximum speed of effectiveness, and other such aerodynamic similarities. Piqued my interest enough to start studying rifle cartridges and bullet design. That led to an interest in rifling, then powders. All my interest in cartridges and bullets eventually led to an interest in which guns matched well with those projectiles. I acquired my first rifle, a Ruger 77 chambered in .257 Roberts. So on and so on.
So similarities to airplanes got me started.