Who am I to write this?
First things first; I know a thing or two about guns. I grew-up in the '80s in rural Missouri in the middle of sporting gun culture. In college I was on my school's (air) rifle team. I qualified as expert in the US Air Force on the M-4 rifle and on an Army course of fire on the M-9 pistol. This post is about writing firearms from a character perspective with an emphasis (but not exclusively) on dialogue. It should not be read from a hobbyist or political perspective.
It's all about your character's background
Before you decide what to arm your character with you need to know a few things about them:
How much do they know about guns?
Did they grow-up around guns or does their knowledge come from their profession (military or law enforcement)?
Are they a collector, hunter, 2A (Second Amendment) absolutist?
Are they a veteran? (If not, do they want to be?)
How old are they?
If you're using a character building tool or even scratching their background down on a post-it, these are questions you should answer before deciding to give them a gun.
All of these should effect how your character handles and talks about their firearms. For example a civilian may call it a gun while a veteran may call it a weapon. Keep in mind that these are not hard and fast rules because human beings are incredibly diverse and stubborn creatures and the answers to these questions will provide contradictions. For example, I have one friend who grew-up in a house that hated guns and became a combat engineer in the Army. He would cringe if anyone ever used clip instead mag or magazine.
On the flip side one of my uncles was drafted into the infantry for Vietnam and only did his allotted time. He grew-up hunting and used the term clip. His two or three years of military service and training was not strong enough to undo his lifetime of civilian gun culture socialization.
Clip or Mag?
Doesn't "clip" and "mag" mean the same thing?
If we're technical about it: NO. This can be like nails on a chalkboard for a reader who is knowledgeable about firearms. The two things, while related, are not the same thing and you're probably going to get some nasty comments and reviews by gun pedants who will assume you don't know the first thing about guns. This is especially true in narration. You might be able to get away with it in dialogue (there are times I'd recommend it); but you refer to a magazine as a clip in narration be ready to get roasted.
Let's start with magazines or mag for short. There are three types: detachable, internal, and tube. A magazine is the part of the gun that holds ammunition until it is chambered for firing. For most modern automatic and semi-automatic firearms a detachable magazine is the norm. This means that the mag can be removed and replaced. This allows for easier and faster reloading.
Older semi-automatic firearms, such as the SKS, may have an internal and/or fixed magazine. These are magazines that cannot be removed without disassembling the firearm. These are also common in bolt action rifles.
The last type of magazine, the tube magazine, is a common feature on sporting rifles and shotguns. These hold ammunition under the barrel. They are especially common in pump shotguns and lever action rifles. Although some semi-automatic sporting rifles use them too.
Clips on the other hand are used to store ammo outside the gun and make loading/reloading easier. For most modern rifles, you shouldn't have to describe this unless your character is armed with the aforementioned SKS which uses a clip to load its internal magazine.
The only time you'd use "clip" in narration is if you're describing stored ammo such as someone showing off their collection at home. Or reloading magazines at a range.
There are times I think you can, and should, use "clip" instead of "mag" and that's in dialogue. It's a way of showing a character's background instead of telling it. For example:
Someone who grew-up with guns as a tool for hunting or shooting predators preying on their livestock may use "clip". (This happened to me just the other day.) This person knows a lot about the practical use of firearms but isn't so obsessed that they have to show everyone how much they know.
On the flip side, in the 1990s after the 1994 Assault Weapons Ban was enacted the AR-15 went from being unpopular with civilian shooters to perhaps the most common sporting rifle today. With the rise of gun forums on the Internet, civilian shooters began mingling with veterans and active duty military. As a result, many civilians became very fussy about calling things by the correct nomenclature. Therefore you can have a character correcting everything other characters say (It's a magazine, NOT a clip dumbass!) as a means of compensating for their real or perceived lack of knowledge.
Finally, they could just be old. My father still uses "clip" because that's what civilians called detachable magazines until the mid-1990s. So if your character was born before the mid-1970s and never served (or just did one tour in 'Nam and GTFO) it's probably inaccurate for them to say anything other than "clip".
NOTE: Revolvers have cylinders; not magazines or clips.
About Assault Rifles or are they "Assault Weapons"...
Another thing that's sure to get you roasted from people who know their shit about guns is the use and misuse of the terms "assault rifles" and "assault weapons". There is a lot to unpack here and it would be easy to delve into the weeds. It's best to just bullet point it:
According to legend assault rifle came about in WWII when the Maschinenpistole 44 (MP 44) was renamed the Sturmgewehr 44 (StG 44). Sturmgewehr translated to assault rifle.
An assault rifle has a very specific definition: fires an intermediate round, has a detachable magazine, and is capable of select fire. This is typically used only by law enforcement and the military. Owning these legally requires an extensive background check and the firearms themselves are very rare because none made after 1986 can be sold to civilians. So unless your character is very rich (and stupid) or is an outlaw they're probably going to be using a semi-automatic version of the military rifle. Here's an Army training video on the AK-47 where the term is defined.
The term assault weapon is a legal, not a technical term. There is no single definition of what it is because it is defined by different levels of governments when they drafted a specific law. What Missouri, California, Anchorage, or Chicago consider an "assault weapon" will be different. Therefore I recommend ONLY using this term in dialogue and NEVER narration. "Military-style rifle", "semi-automatic rifle", or "gun" would be far better and save you grief. If you're using it in dialogue, only use it by someone who is a novice, favors gun control, hates guns, or generally doesn't know anything about firearms.
The AR in AR-15 means Armalite Rifle not assault rifle. In the 1950s the US Army approached a company called Armalite to develop rifle shooting a small caliber rifle cartridge based on a previous design the company submitted. This became the AR-15 which in 1964 was designated the M-16 by the US Air Force. By this time Colt owned the rights to the gun and decided to market a civilian version called the AR-15 SP1. Since then it's become common to refer to the select fire version as the M-16 (or the smaller M-4) and the semi-auto version as the AR-15.
While it's counterintuitive; military firearms are not the most deadly guns on the market. According to the Law of Armed Conflict (LOAC), military small arms (handguns and rifles) are prohibited from making death inevitable. Arming a character with a military style rifle and then having them quickly and easily dispatch ludicrous amounts of enemies is a cliché you should avoid. Things like full-auto and 3 round burst fire are designed not kill a lot of people at one time but rather keep an enemy pinned down so your troops can flank them and then convince them to stop fighting. If you want to give your character something truly and gruesomely lethal skip the AR or AK and go to a 12 gauge shotgun. At close range these are the most lethal firearms you can get. Second to this will be a handgun; it's the preference for most criminals because they're easier to conceal and paired with the right bullet incredibly deadly. For long range go with a high-power hunting rifle.
There is a small but vocal part of the shooting community that hold to the myth that no such thing as an assault rifle exists. A common, but grammatically misguided reasoning, is that "assault" is a verb not a noun and that an inanimate object cannot perform an action. What they're forgetting is that action verbs like "assault" can be used as a subject in a sentence instead of a noun: We launch the assault at dawn. In addition to historical evidence, they're also blissfully ignoring the existence of compound nouns such as race car or house party.
So what are some examples of using these terms? Assault rifle isn't really used that much in the military anymore. So, it's somewhat archaic. People just refer to the M-16 or an AK as a rifle. If you're character joined the military after 2000, just use rifle. I've had young troops attempt to correct me when I've uttered the term, these troops then got a history lesson as thanks for their attempted "help".
Like "clip" older shooters may use the term assault rifle to refer to the semi-auto versions of these firearms. While technically incorrect, in the late '70s and early '80s these firearms were marketed as assault rifles despite being semi-auto versions. When I was in middle school I would flip through the pages of Guns & Ammo Assault Rifle or Gun Digest: Complete Book of Assault Rifles.
These were not select-fire firearms but semi-auto versions reviewed for the civilian collector and/or survivalist. There was even a store on the outskirts of Saint Louis called Assault Systems and it specialized in military style firearms. If you went in and asked to look at an assault rifle you weren't corrected. An older shooter born before 1970 may still call an AR-15 SP1 an assault rifle and be fairly knowledgeable; just not current on what's acceptable nomenclature on social media and internet forums.
For further reading...
The Writer's Guide to Weapons: A Practical Reference for Using Firearms and Knives in Fiction by Benjamin Sobieck is a great resource for how to properly use and describe weapons in your fiction. Available at Amazon and Barnes & Noble.