There are many "uncommon" mistakes, usually having to do with the details of some obscure weapon or other. Those are somewhat forgivable, if still undesirable.
But the common mistakes aren't so forgivable because they occur in spite of some very basic knowledge. There are two of these that seem to happen more frequently than anything else. The first of these common mistakes is using "clip" instead of the proper "magazine."
This is a clip:
These are magazines:
A clip is what is used to load a magazine. Easy rule of thumb: if the device has a spring, then it's a magazine. If not, then it's a clip.
The second common mistake had to do with safeties. I've read a number of books where the hero flicked the safety off his Glock.
Glocks don't have manual safety levers. Neither do revolvers. There may be rare exceptions to this, of course--there's lots of variation among firearms, as well as custom builds--but they are rare. If your hero flicks the safety off his Glock, or, heaven forbid, off his revolver, then some of your readers are going to roll their eyes. They'll think you've never seen a gun other than on the silver screen, much less actually fired one. And you don't want your readers rolling their eyes at your work.
So instead of inserting a fresh clip in his Glock and then thumbing down the safety, have your character insert a fresh magazine and then release the slide. Your readers will appreciate it.