I understand some people (more so the people that rent out shops to get paid. Tattoo artists, or if you prefer, tattoo humans.) may refer to said device as a "tattoo machine", however a "tattoo gun" being used in reference to the device shouldn't be so uncommon that it requires grammatical correction. Even especially when someone makes a short statement, which is more than likely using actual slang terminology (it can usually be recognized when someone starts the sentence like "pulled out the.." Or maybe it'd been more noticeable had I said "Yep, I done went and pulled out the 'ol tattoo gun, yessiree! Why I sure did! That 'ol contraption, why I tell you, heh heh, sure did keep all thems varmints away last night makin' all that ruckus! Yep, it sure did I tell yeh. Yep.". However that was way too much to type. No offense but depending on who you are and what you like to refer to it as, neither are incorrect. It can be referenced to by either. But it doesn't sound very charismatic calling it that for me. I'll continue to say "gun" you can call it whichever you like. But it isn't incorrect either way so no need for corrections. No disrespect intended to you but I'm not fond of being corrected, especially when a tone of conversation can dictate many different aspects of dialect divarication.