I assume they reserve the unicode character, but anyone who wants to use it decides what it looks like (so the rifle could look different on different platforms, which isn't a big issue)
This is always true for Emoji. The platforms always decide what their presentation of emoji will look like, just as they determine what font they will use for the unicode traditional letters. In this case, Apple and others decided they didn't want a rifle emoji, so it was moved to the 'black and white symbols' section so that those platforms wouldn't be 'missing' an emoji, which has other technical implications.
