I have noticed the same thing in his Prey series. He makes a number of small firearm mistakes about handguns. For instance, in " Sudden Prey" he has his BG count the 6 rounds in his 44 cal. Charter ( 5 shot) bulldog while waiting in ambush. He is also apparently unaware that semi auto pistols lock back when the magazine is empty as seen in the hospital shoot out near the end of the book where he has Del's pistol going " click, click" when his magazine is empty. Not to mention the odd situation in the opening part of the book where he has - If memory serves- five experienced police detectives in a van, each of whom has a different type and caliber handgun- but no long guns or vests- preparing to face what they know to be two armed bank robbers.
However, he does seem to be on firmer ground when talking about rifles and shotguns. I suspect he does some hunting. And he does do research - or have an assistant do research- in an effort to give his characters distinctive handguns ( P7, bulldog, Colt Commander, ) rather than relying on a generic "auto". So, he is at least trying to get it right, which is much more than most writers do. Given the generally low standard of gun knowledge in fiction, I find that very admirable.