Someone will probably disagree with this advice, but my suggestion is, if you don't know much about guns and ammo, it's safer to keep things vague and generic, so you don't risk getting the details wrong. Unless you're writing a police or medical report, for example, you don't really need to specify what kind of gun is used, or what caliber it was.
If you do decide to get specific, make sure you get a knowledgeable beta-reader to look the story over. Find someone who knows about guns and ammo, so they can read the scene and find any mistakes. That's what I did when I had to write a scene at a shooting range. I got most of the scene right, but there were a couple of details I got wrong. If I hadn't had a good beta-reader, those mistakes would still be in the novel.