"Fill your hand, you son of a bitch!" Rooster Cogburn (John Wayne).
Let's talk guns, but first.... I know some of this is intended to be funny (you can judge how successful I am) but people with guns have caused unbelievable tragedy. Police officers in the United States carry an array of guns and a writer needs to research the subject if they have cop characters. Just.... We may joke around about our guns, our shooting prowess and everything else but people being shot is a terrible thing. Treat that cautiously.
"I got my nine and my money and my money and my nine."
Police officers are gun people. How immersed, conversant and dedicated is a matter of degree, but we all know and respect firearms. They are tools but also confer bragging rights, engender friendly competition and sometimes break the family budget. We have:
Duty weapon - the primary sidearm an officer carries. It isn't a "piece," "rod," "hog's-leg" or any of the wonderful Hollywood expressions. If you have your cop character "Whipping out her rod" it better be to get a laugh. Most are 9mm caliber, but aficionados will swear by .45s on the theory that size matters (which is a wonderful double entendre, especially coming from a woman). Don't ever have your officer rack the slide for effect. If there isn't already a round in the chamber they are an idiot. Oh.... The bullets are carried in magazines, not clips.
Back up - a smaller pistol, hidden. Anything from a five-shot .38 to a .380 semi-auto. Detective James Carter (Chris Tucker) uses his backup in Rush Hour, idiotically disarming himself but saving the day with an especially athletic move. Me.... I'd stay away from cool moves, but having a second weapon is always a good idea.
Shotguns - now we're talking. Again...scattergun, master-blaster, double ought nightmare and other cute names are just that. Too cute. A shotgun with 00 buckshot is a nasty, serious weapon meant to cut down everything in it's path. The sound of the slide being worked is often enough to send a chill up everyone's spine. These weapons are typically carried without a round in the chamber so the well-timed rack-rack works here. Just don't overuse it. They kick like a mule, so a day at the range leaves most people sore. I know - somebody out there is saying "I know how to shoot the damn thing, I'm not sore." Good for you, big guy.
Rifles - presumed to have a very high CDI ("chicks dig it") factor. Although some agencies carry the Rugar Mini-14 (a wonderful weapon in its own right) you can't go wrong sticking with some variant of the semi-auto AR-15. The fun part of this system is proper accessorizing. True believers exist for any of the components - spirited "discussions" about the relative virtues of optic choices occur out in the real world. Is your character an EOTech fan, an Aimpoint guru or, maybe one of the other products out there? Cheaper Than Dirt will help get you started. More toys than time to discuss exist for this firearm, but let's be serious for a moment. Regardless of what you read in the context of the "assault rifle ban" arguments, the rifle I carry at work is designed for one purpose - to give me an overwhelming advantage in a gun battle. It isn't really a plaything. A writer can play with it, but in the hands of an officer it isn't a "precision sporting instrument" as I've heard it described. It's a tool of combat, meant to "neutralize the threat" as quickly and efficiently as possible and we all know what that means.
Other stuff - the straight piece of wood we carry is a baton, not a "billy club." Most of us have knives, some of them quite impressive -it makes opening that non-Paleo two AM bag of chips easier.
Most of us are competent, capable shooters who have embraced the tools of our trade. We handle weapons with calm efficiency, keep them clean and when called on can wield them with deadly effect. What your officer thinks about their weapons says something about them. A good cop knows at least enough about guns to handle many of them well, although.... Ours is a social science. Don't get too carried away.
Two of my three main chartacters were SWAT cops and that is a whole other matter. If you have a SWAT cop for a character do what I do - have several on speed dial and ask questions.