Guns are a tricky line to walk in my opinion; being a major prop in any military theme they say a lot and need to be pretty consistent. If not, they distract from the overall look of the character and setting becoming a thorn later on.
It's just as good to set ground rules for guns as any part of design as not to stray from what becomes your main identity.
To start I'll break down how I draw guns. In reality they're really simple and only become complex when you start doing odd angles or complex shapes. (If you begin to get super realistic about it's engineering side then it becomes a bit hellish.)
I'll use an AK as a base for this example, as they're extremely clear in silhouette. As you can see I break down the parts within boxes. Mainly three as this is how the character's hands interact with the gun: Pistol grip, body & magazine. If I'm designing from scratch I'll go even further and lay out specific places. Barrel position, the trigger guard etc. It's a pretty effective way to simply layout how the gun is drawn and gives a reference for future drawings. As you can draw this series of squares from multiple angles.
This gives the SHAPE of the prop. How it's held and how it's gonna be shot. This is CRITCAL in drawing these. Just a simple parti of it's body should be enough for any artist to see how the character is gonna carry it around during the early stages of sketching.
The tricky part now comes in when you begin to design your own gun. However the same principles still apply.
It's good to start developing a language for your "Props" or guns in this situation. I personally have wrestled with this throughout the past year a lot. But I have settled on semi-traditional firearms with the addition of aggressive modding.
I feel like it's a good mix of reality with a nice bit of edge that gives the cyberpunk look it's glow. Too bulky and you get Bolters from Warhammer, too fancy and it becomes tacky & scifi looking. Too Mundane and it just becomes another 'tacticool' Tarkov Meta M4 some spats wearing anime woman is gonna carry. (Not that there's anything wrong with the last one. I've drawn my handful and enjoy those a lot during the right occasion.)
Just find your sweet spot and make it your own.
Even then a lot of the hard work is done for you, there's plenty of real oddballs out there that only made it to the prototype phase. Even some modern weridos that have an alien shape because they're not commonly seen. Real unicorns of design.
A few examples of other artists doing such is The Storm Rifle from the Citadel (Beyond). A real beautiful illusion to the Chinese CQZ.
Real design with minor changes, the best part is that it works too. It's not complicated because someone with a history of engineering has already done the heavy lifting better than you.
What comes next after picking a design can go multiple ways. Sometimes it's best to leave it relatively close to the original but you can mangle it into something more fitting if need be. Just taking a handful of parts and you can work a new form into existence. It's what happened with movies like Aliens, Star Wars and Bladerunner for it's prop design.
It just takes a bit of research and adjustments to get it right. Your design doesn't have to make sense always, it has to work with the setting and character more than anything. Is it a quiet pistol with a heavy silencer? A bulky machine gun toted by two big metal hands? Discover what kind of dialogue needs to happen between the character and their prop first.
GITS (Movie) for example runs with it's tight fitting suits and laser focused Major to carry these elegant curves of polymer and these sharp edges of black steel.
Don't make it too complicated, remember that it's gonna be the character carrying this thing. So think about how it's gonna be held and positioned by them. Guns have weight and are awkward until they're held tight to shoot.
How it's shaped can bring a lot more substance than if the gun is realistic. You can go into that design territory afterwards; as that’s just as much of a fun part when making a gun. But If you want to go that hyper realistic route make sure you get a damn good design first.
I’m mainly here to make unique looking props that go well with what I draw. Not H&K’s next big time project.
To end off, shape and silhouettes mean a lot when it comes to props. Think about it next time you see a character holding a specific model of gun & why the artist & prop-designers might choose it specifically.