Originally Posted by MadMorbius
Because that's not what the word means. If I have a red car, I write red car because "Red" is the adjective, and car is the noun. I wouldn't say I drive a red, because that doesn't make sense.
A rifle is a noun. A person, place or thing. A "sniper" is also colloquially a noun, but it more properly an adjective when used in conjunction with another noun. Using it by itself changes the context of the entire sentence, which is incorrect.
Why is this so difficult to understand?
I understand what you are saying, but why is it so difficult to understand someone if they say that they just got a sniper. Would you think they are talking about the rifle or a person that they purchased off ebay? I would think its common sense.
Its all part of the evolution of language after all.