I've heard too many people describe SMGs and Assault Rifles as "Machine Guns" just because they're fully automatic and "Not a Pistol or like Grandpappy's Rifle".
Remember, this is some guy whose only experience with firearms is what he's seen on the 'Trid and heard on the news (Where, I drek you not, they reported about a "Fully-Automatic Bolt Action Rifle" once.). He didn't even know about the idea of a manual safety to start looking for it, so, writing from his perception...
When we switch to Jon "Money" Johnson, we don't just get the right designation, we get the make and model of weapon. If he has time to examine it, he might even give the year. It's one of his hobbies.
Nas also calls his Machine Pistols "Machine Guns" as well, despite being only Ceska Black Scorpions. But he gets pistol and rifle designations correct due to his Redneck upbringing. He can tell a semi-auto rifle from a bolt-action from a lever-action at a glance, where your average city dweller would say, "That's a Rifle, right?" and be happy with that.
Firearm and Gun nomenclature can be very exacting, and Gun Nuts like myself can be quite nitpicks about them (It gets worse if said Gun Nut served in the military!), but I also know that most of the average people don't know these things. So, I write with that in mind.
Thank you for the comments. I like showing the work that goes into how I write.