Dean Koontz
About Koontz, IIRC, he made a fair amount of blunders early on in his career, but it's been so long ago that I'm not positive. I just remember thinking "that's not right!" when I was reading some of his stuff.
Stephen King - really poor firearms knowledge. That's really too bad, because he's one of my favorite authors (especially his earlier works... 'Salem's Lot, The Stand, IT (one of the best novels ever written as far as I'm concerned). It seems to me that he's lost it around the time he published Gerald's Game).
I agree with posters about Hunter and Sanford: much better than average. I REALLY like Stephen Hunters
Pale Horse Coming with all the barely disguised old-time gunnies in it. FWIW, the early part of the novel seemed to go on forever until Earl finally escaped and it began to grab my attention.
Ahern: obviously a gun person. The Survivalist series was good; the (too short) Takers series was better from a plot standpoint. He went into a lot of weapon detail. Books were a quick read (and in retrospect not as good as I thought originally. I bought the first five of The Survivalist through an on-line used bookstore about a year ago and enjoyed them, but not to the extent I remembered. In fact, I quit about 1/2 way through the fifth one.) Of course, he now owns/runs the New Detonics... so I gotta give him credit for that.
John Ringo: top-notch. I haven't seen any mistakes yet in the Ghost series. I just recently read the published books and notwithstanding the sex in the first one (which really didn't bother me) they are pretty good.
John Ross and Matthew Bracken: perfect score IMHO. But... they're part of us, so they should be.
Don Pendleton - read the Executioner back in the 70s when they came out. I really liked the gun stuff, but didn't know as much as I know now.