You're Kidding Me, Right..!!..?
I don't agree that non firearms people use such words.
WHAT..!!..? With due respect, I can only conclude that you've lead a very sheltered life with NO access to T.V., newspapers, magazines, radio, etc., etc. having NO human interaction while living a 1,000 miles out in the middle of nowhere.
I know plenty of people knowledgeable of firearms who use all three examples.
Which just goes to show (and strengthen) my point. If a person who's knowledgeable in a subject misuses words about that particular subject, how can you come to the conclusion (as in your first sentence of your post), that a non-knowledgeable person talking about that SAME subject would know the correct terms/word usage. To me your conclusion makes NO sense.
Automatic used to be the accepted term, and was replaced with "Semi-automatic" at a later date. I prefer Automatic and Machine gun. I don't much care for Semi-automatic.
What was done in the past isn't always correct. Look at Slavery as a example. Even though it was done in the past, it doesn't mean it was the right thing to do.. and in some cases it wasn't acceptable by others.
As far as "automatic" and "machine gun"? Even THAT may be incorrect if your talking about a "one pull of the trigger automatically fires all rounds" type firearm if it's in a pistol caliber. In that case it's a "sub-machine" gun or "machine pistol".
As far as words like clip and bullet (for ammunition) as a writer of fiction I'd probably use such words in certain situations to reflect reality, although I don't use them myself.
I don't disagree with your right to use them.. but personally I get turned off when reading a book or magazine article (especially when the author has previously shaped and defined the book or magazines main character as being a 100% firearms expert with over 40 years experience as a example), who fired his 19 caliber automatic revolver a dozen and a half times (at which point two vehicles blew up in a hundred thousand pieces and one building a mile and a half away was instantly destroyed), before reloading it with the attached clip.
Anyways.. Good Luck with your future story writing. Some day I might be able to tell my grand kids about the "Pulitzer Prize" author named "Hammerklavier" that I once had a spirited discussion with over the internet.
My Best..
Single Action Six