Strongly identify with kjtrains buying ammo first; something I have done multiple times in my gun collecting. In fact, I bought a box of 10MM before I got the Glock 29.
I was motivated by the Bren Ten; a gun I pursued but never found. When Glock came out with the model 29, I decided to see how the cartridge performed and enjoyed it for a few years.
I am also fascinated with old hyphenated cartridges and I bought a box of 25-35 prior to finding a vintage Model 55 Winchester in that caliber. Ditto, .327 magnum, before I found the Ruger SP101, which, at the time, was new to the market.
Buying ammo for a gun you do not own is kind of like buying a suit to match your tie. However, by purchasing the ammo for a gun not yet owned, I found it a mechanism to channel the purchase of the gun "to fit the ammo already purchased"; wink, wink.