I think Long Colt is a descriptive nickname to more positively identify the cartridge vs the 45 Automatic Colt. Slang is sometimes more practical than the dictionary. The correct adjective for "easy to catch on fire" is inflammable. Yet in the hazmat lingo it is often flammable, a pidgin English word, so the folks won't mistake it for the opposite. I used to say 45 Long Colt all the time, so the guy at the counter would not ask me back "which one."